[flang][driver] Add -Xflang and make -test-io a frontend-only flang

This patch adds support for `-Xflang` in `flang-new`. The semantics are
identical to `-Xclang`.

With the addition of `-Xflang`, we can modify `-test-io` to be a
compiler-frontend only flag. This makes more sense, this flag is:
  * very frontend specific
  * to be used for development and testing only
  * not to be exposed to the end user
Originally we added it to the compiler driver, `flang-new`, in order to
facilitate testing. With `-Xflang` this is no longer needed. Tests are
updated accordingly.

Differential Revision: https://reviews.llvm.org/D96864