[flang] MERGE result is polymorphic only if TSOURCE and FSOURCE are polymorphic

16.9.129 point 4: the result is polymorphic if and only if both TSOURCE and
FSOURCE are polymorphic.

If neither TSOURCE and FSOURCE are polymorphic then the current behavior is
preserved.

Depends on D145058

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D145060