[clangd] Get rid of AST matchers in CodeComplete, NFC

Summary:
The isIndexedForCodeCompletion is called in the code patch of
SymbolCollector.

Reviewers: kadircet

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

Differential Revision: https://reviews.llvm.org/D55206

llvm-svn: 348130