[fir] Add fir.select_type conversion placeholder

As for D113662, this patch just add a place holder for
the `fir.select_type` operation conversion. This operation
is part of F20xx and is not implemented yet.

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

Reviewed By: kiranchandramohan

Differential Revision: https://reviews.llvm.org/D113878