Try to fix generation of LLVMExports.cmake under Visual Studio.

Try to fix generation of LLVMExports.cmake under Visual Studio.
If LLVMDebugInfoPDB links against the DIA SDK then the exports file
would contain an INTERFACE_LINK_LIBRARIES property that contained an
absolute path to ``diaguids.lib`` which used a native windows path (interpreted
as escape sequences when LLVMExports.cmake is imported causing
``find_package(LLVM)`` to fail) rather than the correct CMake style path.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240181 91177308-0d34-0410-b5e6-96231b3b80d8