Look for libstdc++ first on the clang install directory.

When sysroot is not set, look for libstdc++ first on the clang install
directory. Before this change if clang was installed alongside a gcc with
the same version as the system one we would select the system libstdc++.

Unfortunately this is hard to test as only the non-sysroot case is changed.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189536 91177308-0d34-0410-b5e6-96231b3b80d8