Sema: Don't reuse PreparedOverload from normal type checking in salvage()

We want to re-prepare our overloads, since now they will contain fixes.

include/swift/Sema/Constraint.h

@@ -883,11 +883,7 @@ public:
     return Overload.Prepared;
   }
 
-  void setPreparedOverload(PreparedOverload *preparedOverload) {
-    ASSERT(Kind == ConstraintKind::BindOverload);
-    ASSERT(!Overload.Prepared);
-    Overload.Prepared = preparedOverload;
-  }
+  void setPreparedOverload(PreparedOverload *preparedOverload);
 
   FunctionType *getAppliedFunctionType() const {
     assert(Kind == ConstraintKind::ApplicableFunction);

include/swift/Sema/ConstraintSystem.h

@@ -4876,7 +4876,8 @@ public:
   /// Build and allocate a prepared overload in the solver arena.
   PreparedOverload *prepareOverload(OverloadChoice choice,
                                     DeclContext *useDC,
-                                    ConstraintLocator *locator);
+                                    ConstraintLocator *locator,
+                                    bool forDiagnostics);
 
   /// Populate the prepared overload with all type variables and constraints
   /// that are to be introduced into the constraint system when this choice

include/swift/Sema/PreparedOverload.h

@@ -160,7 +160,11 @@ public:
 private:
   Type OpenedType;
   Type ThrownErrorType;
-  size_t Count;
+  unsigned Count : 31;
+
+  /// A prepared overload for diagnostics is different than one without,
+  /// because of fixes and such.
+  unsigned ForDiagnostics : 1;
 
   size_t numTrailingObjects(OverloadToken<Change>) const {
     return Count;
@@ -168,9 +172,9 @@ private:
 
 public:
   PreparedOverload(Type openedType, Type thrownErrorType,
-                   ArrayRef<Change> changes)
+                   ArrayRef<Change> changes, bool forDiagnostics)
     : OpenedType(openedType), ThrownErrorType(thrownErrorType),
-      Count(changes.size()) {
+      Count(changes.size()), ForDiagnostics(forDiagnostics) {
     std::uninitialized_copy(changes.begin(), changes.end(),
                             getTrailingObjects());
   }
@@ -183,6 +187,10 @@ public:
     return ThrownErrorType;
   }
 
+  bool wasForDiagnostics() const {
+    return ForDiagnostics;
+  }
+
   ArrayRef<Change> getChanges() const { return getTrailingObjects(Count); }
 };