Fix r291495 -- Normalize LLVM_CMAKE_PATH in clang standalone build.
Fix r291495 -- Normalize LLVM_CMAKE_PATH in clang standalone build.

CMake handles paths with slashes. It caused cmake/install failure on msbuild.exe.

Note, Other llvm-config-oriented variables have been normalized since they are stored in the cache attributed with PATH.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@294954 91177308-0d34-0410-b5e6-96231b3b80d8

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