[flang][lowering] handle MERGE with different FSOURCE and TSOURCE types

In merge FSOURCE and TSOURCE must have the same Fortran dynamic types,
but this does not imply that FSOURCE and TSOURCE will be lowered to the
same MLIR types. For instance, TSOURCE may be a character expression
with a compile type constant length (!fir.char<1,4>) while FSOURCE may
have dynamic length (!fir.char<1,?>).

Cast FSOURCE to TSOURCE MLIR types to handle these cases.

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

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D128507



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