[clangd] Don't insert extra namespace qualifiers when Sema gets lost.

Summary:
There are cases where Sema can't tell that "foo" in foo::Bar is a
namespace qualifier, like in incomplete macro expansions.

After this patch, if sema reports no specifier but it looks like there's one in
the source code, then we take it into account.

Reworked structure and comments in getQueryScopes to try to fight
creeping complexity - unsure if I succeeded.

I made the test harder (the original version also passes).

Reviewers: ioeric

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60503

llvm-svn: 358091