[clangd] Use the index-based API to do the header-source switch.

Summary:
If the file heuristic fails, we try to use the index&AST to do the
header/source inference.

Reviewers: kadircet

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68211

llvm-svn: 373320