Enhance the clang -v gcc debug printing to skip obviously bad and duplicate paths.

Otherwise it lists all files (e.g. shared libraries) that happen to be in the
same paths the GCC installations usually reside in.

On a x86_64 Debian 7 system with i386 multilibs.
before: clang -v 2>&1|wc -l
        3059
after:  clang -v 2>&1|wc -l
        10

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188400 91177308-0d34-0410-b5e6-96231b3b80d8