A bug report on IRC pointed out that we were giving the -Wabi warning in 
cases that don't affect the ABI at all, namely use of scoped enums in 
switch.  So this patch limits the warning to the varargs case by 
catching scoped enums in perform_integral_promotions so that only 
callers that use the type_promotes_to hook directly will get the warning.

Tested x86_64-pc-linux-gnu, applied to trunk and 4.6.