[AST] Check the signature of a nominal type before forming a conformance.

The introduction of the request-evaluator is causing some conformances
to be formed before the nominal type's signature is being checked,
causing a crash while type-checking the standard library. Ensure that this
cannot happen.

Author: DougGregor

lib/AST/ConformanceLookupTable.cpp

@@ -817,6 +817,8 @@ ConformanceLookupTable::getConformance(NominalTypeDecl *nominal,
   if (auto resolver = nominal->getASTContext().getLazyResolver()) {
     if (auto ED = dyn_cast<ExtensionDecl>(conformingDC)) {
       resolver->resolveExtension(ED);
+    } else {
+      resolver->resolveDeclSignature(cast<NominalTypeDecl>(conformingDC));
     }
   }