[AST] Hack SourceFile::lookupLocalType() to look through local types.

This is only currently exercised by swift-remoteast-test, so do the
minimum to ensure that we’re getting cached mangled names, but don’t
fret over the linear-time search.

include/swift/AST/Module.h

@@ -1052,6 +1052,8 @@ public:
   virtual void
   getPrecedenceGroups(SmallVectorImpl<PrecedenceGroupDecl*> &results) const override;
 
+  virtual TypeDecl *lookupLocalType(llvm::StringRef MangledName) const override;
+
   virtual void
   getLocalTypeDecls(SmallVectorImpl<TypeDecl*> &results) const override;