[clangd] Support indexing MACROs.

Summary: This is not enabled in the global-symbol-builder or dynamic index yet.

Reviewers: sammccall

Reviewed By: sammccall

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

Differential Revision: https://reviews.llvm.org/D49028

llvm-svn: 336553