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/SerializedModuleLoader.h

@@ -132,6 +132,10 @@ public:
 
   virtual TypeDecl *lookupLocalType(StringRef MangledName) const override;
 
+  virtual TypeDecl *
+  lookupNestedType(Identifier name,
+                   const NominalTypeDecl *parent) const override;
+
   virtual OperatorDecl *lookupOperator(Identifier name,
                                        DeclKind fixity) const override;