[flang] Fix AllocaOp/AllocMemOp type conversion

For arrays without a constant interior or arrays of character with
dynamic length arrays, the data types are converted to a pointer to the
element type, so the scale size of the constant extents needs to be
counted. The previous AllocaOp conversion does not consider the arrays
of character with dynamic length arrays, and the previous AllocMemOp
conversion does not consider arrays without a constant interior. This
fixes them and refactors the code so that it can be shared. Also add
the test cases.

Reviewed By: Jean Perier

Differential Revision: https://reviews.llvm.org/D124766