[clangd] Optimize "don't include me" check.
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.
Subscribers: MaskRay, jkorous, arphaman, kadircet, cfe-commits
Differential Revision: https://reviews.llvm.org/D61120