[flang] Fix "not yet implemented" message for CHARACTER MIN/MAX

The check to see if the arguments for the MIN/MAX intrinsics were of CHARACTER
type was not handling assumed length characters.  In this case, the FIR type is
"!fir.ref<!fir.char<1,?>>".

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

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D128922



Co-authored-by: Peter Steinfeld <psteinfeld@nvidia.com>