[clang][Interp] Do r-to-l conversion immediately when returning (#80662)

First, we need to register local constant variables in C, so we get the
same diagnostic behavior as the current interpeter.

Second, when returning an LValue (as a Pointer), which we eventually
convert to an RValue, we need to do the conversion immediately when
saving the Pointer in the EvaluationResult. Otherwise, we will possibly
deallocate the data before doing the conversion (which will look at the
Block*).