[clangd] Refactor speculateCompletionFilter and also extract scope.

Summary:
Intent is to use the heuristically-parsed scope in cases where we get bogus
results from sema, such as in complex macro expansions.
Added a motivating testcase we currently get wrong.

Name changed because we (already) use this for things other than speculation.

Reviewers: ioeric

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60500

llvm-svn: 358074