[MSVCToolChain] Guard hardcoded Windows paths with LLVM_ON_WIN32
[MSVCToolChain] Guard hardcoded Windows paths with LLVM_ON_WIN32

Paths like C:/foo will never work on UNIX platforms, don't bother
implicitly adding them to the search path.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@276606 91177308-0d34-0410-b5e6-96231b3b80d8

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