[CMake] Convert paths to the right form in standalone builds on Windows

The paths output from llvm-config --cmakedir and from clang
--print-libgcc-file-name can contain backslashes, while CMake
can't handle the paths in this form.

This matches what compiler-rt already does (since SVN r203789
and r293195).

Differential Revision: https://reviews.llvm.org/D48353

llvm-svn: 335169