[flang] Compute type allocation size based on the actual target representation.

This change makes sure that we compute the element size and the byte stride
based on the target representation of the element type.

For example, when REAL*10 is mapped to x86_fp80 each element occupies
16 bytes rather than 10 because of the padding.

Note that the size computation method used here actually returns
the distance between two adjacent element of the *same* type in memory
(which is equivalent to llvm::DataLayout::getTypeAllocSize()).
It does not return the number of bytes that may be overwritten
by storing a value of the specified type (e.g. what can be computed
via llvm::DataLayout::getTypeStoreSize(), but not available in
mlir::DataLayout).

Differential Revision: https://reviews.llvm.org/D133508