[Compiler-rt] Broken compiler-rt CMake configuring on Windows

Summary:
Hi Michal,

Would you be able to review this simple fix, please?

Since r291504 compiler-rt uses `llvm-config --cmakedir` to get the path to the LLVM CMake modules.
On Windows this option returns Windows style path with backslashes. CMake treats backslashes as beginning of an escaped character and thus fails to append the path to `CMAKE_MODULE_PATH`.

Reviewers: compnerd, mgorny

Reviewed By: mgorny

Subscribers: compnerd, llvm-commits, dberris

Differential Revision: https://reviews.llvm.org/D28908

llvm-svn: 293195