[ConstraintSystem] Split unviable lookup result storage into candidates & reasons

That makes it easy to process unviable choices the same way as viable
ones and request rejection reasons only when necessary.

lib/Sema/CSDiag.cpp

@@ -791,15 +791,19 @@ void FailureDiagnosis::diagnoseUnviableLookupResults(
   // Otherwise, we have at least one (and potentially many) viable candidates
   // sort them out.  If all of the candidates have the same problem (commonly
   // because there is exactly one candidate!) diagnose this.
-  bool sameProblem = true;
-  auto firstProblem = result.UnviableCandidates[0].second;
+  auto firstProblem = result.UnviableReasons[0];
+  bool sameProblem = llvm::all_of(
+      result.UnviableReasons,
+      [&firstProblem](const MemberLookupResult::UnviableReason &problem) {
+        return problem == firstProblem;
+      });
+
   ValueDecl *member = nullptr;
   for (auto cand : result.UnviableCandidates) {
     if (member == nullptr)
-      member = cand.first.getDecl();
-    sameProblem &= cand.second == firstProblem;
+      member = cand.getDecl();
   }
-  
+
   auto instanceTy = baseObjTy;
   if (auto *MTT = instanceTy->getAs<AnyMetatypeType>())
     instanceTy = MTT->getInstanceType();
@@ -842,7 +846,7 @@ void FailureDiagnosis::diagnoseUnviableLookupResults(
     }
         
     case MemberLookupResult::UR_Inaccessible: {
-      auto decl = result.UnviableCandidates[0].first.getDecl();
+      auto decl = result.UnviableCandidates[0].getDecl();
       // FIXME: What if the unviable candidates have different levels of access?
       //
       // If we found an inaccessible member of a protocol extension, it might
@@ -853,8 +857,8 @@ void FailureDiagnosis::diagnoseUnviableLookupResults(
       diagnose(nameLoc, diag::candidate_inaccessible, decl->getBaseName(),
                decl->getFormalAccessScope().accessLevelForDiagnostics());
       for (auto cand : result.UnviableCandidates)
-        diagnose(cand.first.getDecl(), diag::decl_declared_here, memberName);
-        
+        diagnose(cand.getDecl(), diag::decl_declared_here, memberName);
+
       return;
     }
     }
@@ -4206,7 +4210,7 @@ bool FailureDiagnosis::diagnoseMethodAttributeFailures(
 
   SmallVector<OverloadChoice, 2> choices;
   for (auto &unviable : results.UnviableCandidates)
-    choices.push_back(OverloadChoice(baseType, unviable.first.getDecl(),
+    choices.push_back(OverloadChoice(baseType, unviable.getDecl(),
                                      UDE->getFunctionRefKind()));
 
   CalleeCandidateInfo unviableCandidates(baseType, choices, hasTrailingClosure,

lib/Sema/CSGen.cpp

@@ -3719,7 +3719,7 @@ swift::resolveValueMember(DeclContext &DC, Type BaseTy, DeclName Name) {
 
   // Keep track of all the unviable members.
   for (auto Can : LookupResult.UnviableCandidates)
-    Result.Impl.AllDecls.push_back(Can.first.getDecl());
+    Result.Impl.AllDecls.push_back(Can.getDecl());
 
   // Keep track of the start of viable choices.
   Result.Impl.ViableStartIdx = Result.Impl.AllDecls.size();

lib/Sema/CSSimplify.cpp

@@ -3933,10 +3933,10 @@ retry_after_fail:
           result.addViable(
             OverloadChoice::getDynamicMemberLookup(baseTy, decl, name));
       }
-      for (auto candidate : subscripts.UnviableCandidates) {
-        auto decl = candidate.first.getDecl();
+      for (auto index : indices(subscripts.UnviableCandidates)) {
+        auto decl = subscripts.UnviableCandidates[index].getDecl();
         auto choice = OverloadChoice::getDynamicMemberLookup(baseTy, decl,name);
-        result.addUnviable(choice, candidate.second);
+        result.addUnviable(choice, subscripts.UnviableReasons[index]);
       }
     }
   }
@@ -4232,21 +4232,15 @@ ConstraintSystem::SolutionKind ConstraintSystem::simplifyMemberConstraint(
   }
 
   if (!result.UnviableCandidates.empty()) {
-    SmallVector<OverloadChoice, 8> choices;
-    llvm::transform(
-        result.UnviableCandidates, std::back_inserter(choices),
-        [](const std::pair<OverloadChoice, MemberLookupResult::UnviableReason>
-               &candidate) { return candidate.first; });
-
     // Generate constraints for unvailable choices if they have a fix,
     // and disable them by default, they'd get picked up in the "salvage" mode.
-    generateConstraints(candidates, memberTy, choices, useDC, locator,
-                        /*favoredChoice=*/None, /*requiresFix=*/true,
-                        [&](unsigned idx, const OverloadChoice &choice) {
-                          return fixMemberRef(
-                              *this, baseTy, member, choice, locator,
-                              result.UnviableCandidates[idx].second);
-                        });
+    generateConstraints(
+        candidates, memberTy, result.UnviableCandidates, useDC, locator,
+        /*favoredChoice=*/None, /*requiresFix=*/true,
+        [&](unsigned idx, const OverloadChoice &choice) {
+          return fixMemberRef(*this, baseTy, member, choice, locator,
+                              result.UnviableReasons[idx]);
+        });
   }
 
   if (!candidates.empty()) {

lib/Sema/ConstraintSystem.h

@@ -873,11 +873,12 @@ struct MemberLookupResult {
     /// The member is inaccessible (e.g. a private member in another file).
     UR_Inaccessible,
   };
-  
-  /// This is a list of considered (but rejected) candidates, along with a
-  /// reason for their rejection.
-  SmallVector<std::pair<OverloadChoice, UnviableReason>, 4> UnviableCandidates;
 
+  /// This is a list of considered (but rejected) candidates, along with a
+  /// reason for their rejection. Split into separate collections to make
+  /// it easier to use in conjunction with viable candidates.
+  SmallVector<OverloadChoice, 4> UnviableCandidates;
+  SmallVector<UnviableReason, 4> UnviableReasons;
 
   /// Mark this as being an already-diagnosed error and return itself.
   MemberLookupResult &markErrorAlreadyDiagnosed() {
@@ -890,7 +891,8 @@ struct MemberLookupResult {
   }
   
   void addUnviable(OverloadChoice candidate, UnviableReason reason) {
-    UnviableCandidates.push_back({candidate, reason});
+    UnviableCandidates.push_back(candidate);
+    UnviableReasons.push_back(reason);
   }
 
   Optional<unsigned> getFavoredIndex() const {