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: https://llvm.org/svn/llvm-project/cfe/trunk@294954 91177308-0d34-0410-b5e6-96231b3b80d8