[clangd] Skip .inc headers when canonicalizing header #include.

Summary:
This assumes that .inc files are supposed to be included via headers
that include them.

Reviewers: sammccall

Reviewed By: sammccall

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

Differential Revision: https://reviews.llvm.org/D47187

llvm-svn: 333188