Reapply [Tooling] Make clang-tool find libc++ dir on mac when running on a...

Reapply [Tooling] Make clang-tool find libc++ dir on mac when running on a file without compilation database.

This reverts commit r351282, and re-lands r351222 and r351229 with the
use-after-free fixed.

llvm-svn: 351316