Introduce '-fmodules-user-build-path' which accepts the "canonical" path to a user workspace build.

This is used to avoid conflicts with user modules with the same name from different workspaces.

rdar://16042513

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202683 91177308-0d34-0410-b5e6-96231b3b80d8