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.

include/swift/Serialization/ModuleFile.h

@@ -603,7 +603,7 @@ public:
 
   /// Searches the module's nested type decls table for the given member of
   /// the given type.
-  TypeDecl *lookupNestedType(Identifier name, const ValueDecl *parent);
+  TypeDecl *lookupNestedType(Identifier name, const NominalTypeDecl *parent);
 
   /// Searches the module's operators for one with the given name and fixity.
   ///