[clangd] (take 2) Try harder to find a plausible `clang` as argv0, particularly on Mac.

Summary:
This was originally committed in 88bccded,
and reverted in 93f77617.

This version is now much more testable: the "detect toolchain properties" part
is still not tested but also not active in tests.
All the command manipulation based on the detected properties is
directly tested, and also not active in other tests.

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

Reviewers: kbobyrev, ilya-biryukov

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D71029