Add Multilib selection machinery

This patch improves the support for picking Multilibs from gcc installations.
It also provides a better approximation for the flags '-print-multi-directory'
and '-print-multi-lib'.

review: http://llvm-reviews.chandlerc.com/D2538


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201202 91177308-0d34-0410-b5e6-96231b3b80d8