[flang][hlfir] user defined assignment codegen

Add codegen support for hlfir.region_assign with user defined
assignment.

It is currently a bit pessimistic, because outside of forall, it
does not use the PURE aspect, if any, of the assignment routine to
rule out that the routine can write to something else than the LHS that
could overlap with the RHS.
However, the current lowering is anyway adding parenthesis around the
RHS, so this should not cause performance regressions.

Differential Revision: https://reviews.llvm.org/D153516