[clangd] Try harder to find a plausible `clang` as argv0, particularly on Mac.

Summary:
Fixes https://github.com/clangd/clangd/issues/211
Fixes https://github.com/clangd/clangd/issues/178

No tests - this is hard to test, and basically impossible to verify what we want
(this produces compile commands that work on a real mac with recent toolchain)

(Need someone on mac to verify it actually fixes these!)

Reviewers: kbobyrev, ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D70863