[Conformance checking] Don't suppress substitution failures during checking.

When checking whether a particular protocol conformance satisfies all of
the protocol's requirements, we were suppressing substitution failures.
In some cases, this would mean that we marked a conformance "invalid"
without ever emitting a diagnostic, which would lead to downstream crashes.

Instead, treat substitution failures somewhat more lazily. If we encounter
one while performing the checking, put the conformance into a "delayed" list
rather than failing immediately. Teach the top-level type checking
loop to re-check these conformances, emitting a diagnostic if they
fail the second time around.

Fixes rdar://problem/35082483 and likely other issues that slipped
through the type checker or blew up in unpredictable ways.

lib/AST/ProtocolConformance.cpp

@@ -387,6 +387,11 @@ void NormalProtocolConformance::differenceAndStoreConditionalRequirements() {
 
 void NormalProtocolConformance::setSignatureConformances(
                                ArrayRef<ProtocolConformanceRef> conformances) {
+  if (conformances.empty()) {
+    SignatureConformances = { };
+    return;
+  }
+
   auto &ctx = getProtocol()->getASTContext();
   SignatureConformances = ctx.AllocateCopy(conformances);
 
@@ -460,11 +465,6 @@ NormalProtocolConformance::populateSignatureConformances() {
       other.owning = false;
     }
 
-    ~Writer() {
-      assert((!owning || self->isInvalid() || requirementSignature.empty()) &&
-             "signature conformances were not fully populated");
-    }
-
     void operator()(ProtocolConformanceRef conformance){
       // Make sure we have the right conformance.
       assert(!requirementSignature.empty() && "Too many conformances?");