[flang] Handle BINC(C) variables and add TODO for corner cases

- BIND(C) was ignored in lowering for objects (it can be used on
module and common blocks): use the bind name as the fir.global name.

- When an procedure is declared BIND(C) indirectly via an interface,
  it should have a BIND(C) name. This was not the case because
  GetBindName()/bindingName() return nothing in this case: detect this
  case in mangler.cpp and use the symbol name.

Add TODOs for corner cases:

- BIND(C) module variables may be initialized on the C side. This does
  not fit well with the current linkage strategy. Add a TODO until this
  is revisited.

- BIND(C) internal procedures should not have a binding label (see
  Fortran 2018 section 18.10.2 point 2), yet we currently lower them as
  if they were BIND(C) external procedure.
  I think this and the indirect interface case should really be
  handled by symbol.GetBindName instead of adding more logic in
  lowering to deal with this case: add a TODO.

This patch is part of the upstreaming effort from fir-dev branch.

Reviewed By: klausler

Differential Revision: https://reviews.llvm.org/D128340

Co-authored-by: Jean Perier <jperier@nvidia.com>

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