[clangd] Optimize "don't include me" check.

Summary:
llvm::Regex is really slow, and regex evaluation during preamble indexing was
showing up as 25% on a profile of clangd in a codebase with large preambles.

Reviewers: ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61120

llvm-svn: 359214