[clangd] Don't traverse the AST within uninteresting files during indexing

Summary:
We already skip function bodies from these files while parsing, and drop symbols
found in them. However, traversing their ASTs still takes a substantial amount
of time.

Non-scientific benchmark on my machine:
  background-indexing llvm-project (llvm+clang+clang-tools-extra), wall time
  before: 7:46
  after: 5:13
  change: -33%

Indexer.cpp libclang should be updated too, I'm less familiar with that code,
and it's doing tricky things with the ShouldSkipFunctionBody callback, so it
needs to be done separately.

Reviewers: kadircet

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80296