Summary:
This was originally committed in 88bccded8f,
and reverted in 93f77617ab.
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
Sam McCall