[clangd] Don't insert absolute paths, give up instead.

Summary: Also implement resolution of paths relative to mainfile without HeaderSearchInfo.

Reviewers: kadircet

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64293

llvm-svn: 365364