Set MustBuildLookupTable on PrimaryContext in ExternalASTMerger

[teemperor]

Summary

MustBuildLookupTable must always be called on a primary context as we otherwise
trigger an assert, but we don't ensure that this will always happen in our code right now.

This patch explicitly requests the primary context when doing this call as this shouldn't break
anything (as calling getPrimaryContext on a context which is its own primary context is a no-op)
but will catch these rare cases where we somehow operate on a declaration context that is
not its own primary context.

See also D54863.

[martong]

LGTM. Thank you!

[shafik]

As pointed out in this comment in another review

https://reviews.llvm.org/D44100#1311687

We need to make sure we are running the lldb test suite before committing and watching the bots to make sure the commit does not break them.

Thank you

[davide]

This doesn't break anything, and I'm fairly confident Raphael (@teemperor) ran the tests.

[shafik]

Apologies, I meant to make the comment in the child PR

cfe/trunk/lib/AST/ExternalASTMerger.cpp

if (auto *ToDC = dyn_cast<DeclContext>(To)) {
                  << " maybe recording origin (DeclContext*)" << (void*)FromDC
                  << ", (ASTContext*)" << (void*)&getFromContext()
                  << "\n";
         Parent.MaybeRecordOrigin(ToDC, {FromDC, &getFromContext()});
       }
     }
     if (auto *ToTag = dyn_cast<TagDecl>(To)) {
       ToTag->setHasExternalLexicalStorage();
-      ToTag->setMustBuildLookupTable();
+      ToTag->getPrimaryContext()->setMustBuildLookupTable();
       assert(Parent.CanComplete(ToTag));
     } else if (auto *ToNamespace = dyn_cast<NamespaceDecl>(To)) {
       ToNamespace->setHasExternalVisibleStorage();
       assert(Parent.CanComplete(ToNamespace));
     } else if (auto *ToContainer = dyn_cast<ObjCContainerDecl>(To)) {
       ToContainer->setHasExternalLexicalStorage();
-      ToContainer->setMustBuildLookupTable();
+      ToContainer->getPrimaryContext()->setMustBuildLookupTable();
       assert(Parent.CanComplete(ToContainer));
     }
     return To;
   }
   ASTImporter &GetReverse() { return Reverse; }
 };
 
 bool HasDeclOfSameType(llvm::ArrayRef<Candidate> Decls, const Candidate &C) {