This is a partial revert of b4537c3f51
based on the discussion in https://reviews.llvm.org/D101194. Rather
than using the getMultiarchTriple, we use the getTripleString.
This is useful when looking for directories or files relative to the
toolchain root, e.g. include/c++/v1. This change also adds a test
to make sure this functionality doesn't regress in the future.
Differential Revision: https://reviews.llvm.org/D62558
llvm-svn: 361903