[Runtime] Resolve symbolic references in type mangled name round-trip.

When performing the round-tripping verification for mangled type names,
make sure we resolve symbolic references to something
user-comprehensible that can be meaningfully rem angled.

Part of rdar://problem/37551850.

Author: DougGregor

stdlib/public/runtime/Metadata.cpp

@@ -4688,6 +4688,8 @@ void swift::verifyMangledNameRoundtrip(const Metadata *metadata) {
   if (!verificationEnabled) return;
 
   Demangle::Demangler Dem;
+  Dem.setSymbolicReferenceResolver(ResolveToDemanglingForContext(Dem));
+
   auto node = _swift_buildDemanglingForMetadata(metadata, Dem);
   // If the mangled node involves types in an AnonymousContext, then by design,
   // it cannot be looked up by name.