This is follow-up from a discussion on PR #3998, and a comment by
@tautschnig.
This function always fails, with an exception, when given anything but a
constant_exprt.
This change means that the caller must do the type conversion. The benefit
is to make the caller more aware of the requirement that this must be a
constant, and to make the caller handle the error appropriately (with an
user-friendly error message) in case this is not possible.
The disadvantage is additional code at the call site.
a5944a75a9