[clangd] Parse files without extensions if we don't have a compile command.

Summary: This would enable clangd for C++ standard library files.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63481

llvm-svn: 363663