[MLIR] Normalize path separator in MLIRConfig.cmake (#208659)
Under Windows, the paths may contain `\` path separators.
`MLIR_TABLEGEN_EXE` is added into MLIRConfig.cmake where backslashes are
interpreted as escape characters. This results in syntax errors during
`find_package(MLIR)`.
Fix by using CMake-normalized paths (`/` separator under all platforms).
LLVM/Clang do not put their tablegen paths in their config files.