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.

llvm-svn: 240181