Moving generic constraint from extension to its member or vice versa may change the mangled name
of the member without changing the generic signature, thus introducing ABI breakages. This change
teaches the ABI checker to diagnose USR (mangled name) changes to cover such cases.
rdar://78276290
Before this change, the ABI checker didn't complain about missing @available
attributes on new APIs if the enclosing scopes of new APIs have
availability information. Instead, we should require explicit @available attributes
for new APIs to prevent them inheriting wrong availability information from a
preexisting scope.
rdar://81719628
