Merge r285525 into 3.9.1: Fix clang installed path to handle case where clang...

Merge r285525 into 3.9.1: Fix clang installed path to handle case where clang is invoked through a symlink

See https://llvm.org/PR30840



git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_39@287580 91177308-0d34-0410-b5e6-96231b3b80d8