[clangd] Improve global code completion when scope specifier is unresolved.

Summary:
Suppose `clangd::` is unresolved in the following example. Currently,
we simply use "clangd::" as the query scope. We can do better by combining with
accessible scopes in the context. The query scopes can be `{clangd::, clang::clangd::}`.
```
namespace clang { clangd::^ }

```

Reviewers: ilya-biryukov, sammccall, hokein, kadircet

Reviewed By: kadircet

Subscribers: MaskRay, jkorous, arphaman, kadircet, jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58448

llvm-svn: 354963