[flang] Add TypeAndShape::MeasureElementSizeInBytes()

Split up MeasureSizeInBytes() so that array element sizes can be
calculated accurately; use the new API in some places where
DynamicType::MeasureSizeInBytes() was being used but the new
API performs better due to TypeAndShape having precise CHARACTER
length information.

Differential Revision: https://reviews.llvm.org/D95897