Revert r253898 and r253899, this breaks mingw compilation on openSUSE.

Will re-apply after llvm 3.8 is branched. 

Original commit message:

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

llvm-svn: 257468