[clangd] Use pre-populated mappings for standard symbols

Summary:
This takes ~5% of time when running clangd unit tests.

To achieve this, move mapping of system includes out of CanonicalIncludes
and into a separate class

Reviewers: sammccall, hokein

Reviewed By: sammccall

Subscribers: MaskRay, jkorous, arphaman, kadircet, jfb, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67172

llvm-svn: 371408