[clangd] Handle unresolved scope specifier when fixing includes.

Summary:
In the following examples, "clangd" is unresolved, and the fixer will try to fix
include for `clang::clangd`; however, clang::clangd::X is usually intended. So
when handling a qualifier that is unresolved, we change the unresolved name and
scopes so that the fixer will fix "clang::clangd::X" in the following example.
```
  namespace clang {
  	clangd::X
	~~~~~~
  }
  // or
  clang::clangd::X
         ~~~~~~
```

Reviewers: sammccall

Reviewed By: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58185

llvm-svn: 354330