[flang][hlfir] place scalar in memory in convertToBox/convertToAddress

Implement the TODO. Be careful to use and propagate the expression
type to create the temporary since the mlir value may have been computed
with a different value type (e.g., i1 for logical) that should not be
used for in memory values that must have Fortran types.

Co-authored-by: Tom Eccles <tom.eccles@arm.com>

Differential Revision: https://reviews.llvm.org/D143421