[clangd] Expose whether no-compile completion was used.

Summary: Embedding clients want to experiment with showing such results in e.g. a different color.

Reviewers: kadircet

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61588

llvm-svn: 360039