[clang-tools-extra] Reuse llvm_config.use_clang() to set up test environment

This replaces a bunch of duplicate logic to set up environment variables
and a few substitutions.

It does a little more than we were doing previously:
 - searching for clang and setting up substitutions for it
 - setting up some substitutions for target triples, which are
   potentially useful but not actually used

clangd has been happily using this for its tests for a while.

Differential Revision: https://reviews.llvm.org/D121422