[clangd] Require case-insensitive prefix match for macro completions.

Summary: Macros are terribly spammy at the moment and this offers some relief.

Reviewers: ioeric

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

Differential Revision: https://reviews.llvm.org/D47936

llvm-svn: 334287