[flang] Remove build dependency on clang

For now all we need to depend on is LLVM.
Use llvm_map_components_to_libnames to find libraries to link against.

Original-commit: flang-compiler/f18@dff1402c2a781b7fff5c691b45e6a7f2e3a6ef61
Reviewed-on: https://github.com/flang-compiler/f18/pull/321