[Flang][Driver] Implement OPT_R_Joined options

Add a BackendRemarkConsumer class, responsible for handling diagnostics
received from LLVM. The diagnostics being information on middle and
backend passes used or not used.

Clang by default has all remarks ignored but manually sets the severity of
`R_Group` to visible(`clang::diag::clang::Severity::Remark`). This patch does
the same for Flang.

Depends on D157410. That patch adds the R family of options to
`FlangOption` and `FC1Option` in
`clang/include/clang/Driver/Options.td`

Reviewed By: awarzynski

Differential Revision: https://reviews.llvm.org/D158174