[clangd] Enable extraction of system includes from custom toolchains

Summary:
Some custom toolchains come with their own header files and compiler
drivers. Those compiler drivers implicitly know about include search path for
those headers. This patch aims to extract that information from drivers and add
it to the command line when invoking clang frontend.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62804

llvm-svn: 364389