Driver: fallback to the location of clang if no sysroot,

hard coding /usr makes little sense for mingw-w64.
If we have portable toolchains having /usr breaks that.
If the clang we use is in /usr/bin or /usr/sbin etc this will
still detect as though it was hard coded to /usr

This makes the most sense going forward for mingw-w64 toolchains
on both linux and mac

Differential Revision: http://reviews.llvm.org/D14164

llvm-svn: 253898