clang-cl: Don't look up absolute paths in %LIB%.

Before this patch, passing a non-existent absolute path to clang-cl would cause
stat'ing of impossible paths. For example, `clang-cl -c d:\adsfasdf.txt` would
cause a stat of
C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\LIBd:\asdfadsf.cc

llvm-svn: 235787