clang-cl: implement custom search for cl.exe in /fallback mode
clang-cl: implement custom search for cl.exe in /fallback mode

This solves the problem of fallback onto ourselves if clang-cl
has been renamed to cl.exe and put on the PATH, as happens with
the VS integration.

Differential Revision: http://llvm-reviews.chandlerc.com/D1731

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@191099 91177308-0d34-0410-b5e6-96231b3b80d8

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