[AST] Clear module lookup cache in SourceFile::addTopLevelDecl

Completion unfortunately relies on this to add a top-level decl
after the lookup table has been built, make sure the decl gets added
to it. We ought to fix completion to avoid needing to mutate the AST
like this after-the-fact, but this should work around the issue for now.

Author: hamishknight

lib/AST/Module.cpp

@@ -3495,6 +3495,8 @@ void SourceFile::addTopLevelDecl(Decl *d) {
   auto &ctx = getASTContext();
   auto *mutableThis = const_cast<SourceFile *>(this);
   ctx.evaluator.clearCachedOutput(ParseTopLevelDeclsRequest{mutableThis});
+
+  getParentModule()->clearLookupCache();
 }
 
 void SourceFile::prependTopLevelDecl(Decl *d) {