[flang] Fold MERGE() of derived type values

Generalize FoldMerge() to accommodate derived type arguments and results,
rename it into Folder<T>::MERGE(), and remove it from the various
FoldIntrinsicFunction() routines for intrinsic types.

Fixes llvm-test-suite/Fortran/gfortran/regression/merge_init_expr_2.f90.

Differential Revision: https://reviews.llvm.org/D157345