Revert r253898 and r253899, this breaks mingw compilation on openSUSE. Will re-apply after llvm 3.8 is branched. 
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



git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@257468 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang