Lift nested type lookup fast-pathing up to FileUnit.

We use this to avoid circularity issues in serialization; we'd like to
extend that to the Clang importer. This is only necessary because we
can't look up a single member at a time, but it can still fix issues
in the short term.

This commit should have no effect on functionality.

lib/Serialization/ModuleFile.cpp

@@ -1363,7 +1363,7 @@ TypeDecl *ModuleFile::lookupLocalType(StringRef MangledName) {
 }
 
 TypeDecl *ModuleFile::lookupNestedType(Identifier name,
-                                       const ValueDecl *parent) {
+                                       const NominalTypeDecl *parent) {
   PrettyStackTraceModuleFile stackEntry(*this);
 
   if (!NestedTypeDecls)