[clangd] Collect refs from headers.

Summary:
Add a flag to SymbolCollector to collect refs fdrom headers.

Note that we collect refs from headers in static index, and we don't do it for
dynamic index because of the preamble (we skip function body in preamble,
collecting it will result incomplete results).

Reviewers: sammccall

Subscribers: ilya-biryukov, ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D53322

llvm-svn: 344678