[clangd] Add isHeaderFile helper.

Summary:
we have a few places using `ASTCtx.getLangOpts().IsHeaderFile` to
determine a header file, but it relies on "-x c-header" compiler flag,
if the compilation command doesn't have this flag, we will get a false
positive. We are encountering this issue in bazel build system.

To solve this problem, we infer the file from file name, actual changes will
come in follow-ups.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70235