[flang] Take result length into account in ApplyElementwise folding

ApplyElementwise on character operation was always creating a result
ArrayConstructor with the length of the left operand. This is not
correct for concatenation and SetLength operations.

Compute and thread the length to the spot creating the ArrayConstructor
so that the length is correct for those character operations.

Differential Revision: https://reviews.llvm.org/D108711