c++: Avoid ICE with dependent attribute on type.

c++: Avoid ICE with dependent attribute on type.

We previously happened to accept this testcase, but never actually did
anything useful with the attribute.  The patch for PR86379 stopped using
TREE_TYPE as USING_DECL_SCOPE, so 'using A::b' no longer had TREE_TYPE set,
so the language-independent decl_attributes started crashing on it.

GNU attributes are more flexible in their placement than C++11 attributes,
so if we encounter a dependent GNU attribute that syntactically appertains
to a type rather than the declaration as a whole, move it to the
declaration; that's almost certainly what the user meant, anyway.

gcc/cp/ChangeLog
2020-04-27  Jason Merrill  <jason@redhat.com>

	PR c++/90750
	PR c++/79585
	* decl.c (grokdeclarator): Move dependent attribute to decl.
	* decl2.c (splice_template_attributes): No longer static.