c: fix ICE related to tagged types with attributes in diagnostics [PR120380]

c: fix ICE related to tagged types with attributes in diagnostics [PR120380]

get_aka_type will create a new type for diagnostics, but for tagged types
attributes will then be ignored with a warning.  This can lead to reentering
warning code which leads to an ICE.  Fix this by ignoring the attributes
for tagged types.

	PR c/120380

gcc/c/ChangeLog:
	* c-objc-common.cc (get_aka_type): Ignore attributes for tagged types.

gcc/testsuite/ChangeLog:
	* gcc.dg/pr120380.c: New test.