clang-cl: Fix path-based MSVC version detection
clang-cl: Fix path-based MSVC version detection

The code wasn't taking the architecture-specific subdirectory into
account.

Differential Revision: https://reviews.llvm.org/D33258

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@303267 91177308-0d34-0410-b5e6-96231b3b80d8

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