[flang] Move type.{h,cc} and attr.{h,cc}

The are now in new namespace and directory, "semantics", similar to
"parser".

Original-commit: flang-compiler/f18@115a1341e2f072e19a3a37da85e9f7f91e42c2be
Reviewed-on: https://github.com/flang-compiler/f18/pull/5