Move enum size check into ada_identical_enum_types_p

Currently, the callers of ada_identical_enum_types_p must check that
both enum types have the same number of members.  In another series
I'm working on, it was convenient to move this check into the callee
instead; and I broke this patch out to make that series a little
simpler.

Approved-By: Tom de Vries <tdevries@suse.de>