attribs: fix typedefs in generic code [PR105492]

attribs: fix typedefs in generic code [PR105492]

In my patch for PR100545 I added an assert to check for broken typedefs in
set_underlying_type, and it found one in this case:
rs6000_handle_altivec_attribute had the same problem as
handle_mode_attribute.  So let's move the fixup into decl_attributes.

	PR c/105492

gcc/ChangeLog:

	* attribs.cc (decl_attributes): Fix broken typedefs here.

gcc/c-family/ChangeLog:

	* c-attribs.cc (handle_mode_attribute): Don't fix broken typedefs
	here.