[flang] Replace crash and improve a semantics TODO message

The derived type information table construction code had a
crash whose root cause was replacing an expression with one
of its operands -- the deletion of the LHS of that assignment
led to the RHS being invalidated before it could be read.
Fix by cloning the RHS.  Also update a TODO message to the
new "_todo_en_US" message class and add a comment about how
it should be resolved.

Differential Revision: https://reviews.llvm.org/D126154