Add more pedwarns for [[]] C attributes on types.

Add more pedwarns for [[]] C attributes on types.

The standard [[]] attributes currently defined in C2x are all not
valid on types not being defined at the time.

Use on such types results in a warning from attribs.c about attributes
appertaining to types (the warning that I think is bogus in general
for both C and C++, applying as it does to all [[]] attributes
including gnu:: ones that are perfectly meaningful on types not being
defined and work fine when __attribute__ syntax is used instead).  If
that warning is removed (as I intend to do in a subsequent patch),
warnings may or may not result from the attribute handlers, depending
on whether those particular attribute handlers consider the attributes
meaningful in such a context.  In C, however, the rules about where
each [[]] attribute is valid are constraints, so a pedwarn, not a
warning, is required.

Because some handlers are shared between standard and gnu::
attributes, there can be cases that are valid for the GNU attribute
variant but not for the standard one.  So in general it is not correct
to rely on the attribute handlers to give all required pedwarns
(although in some cases, a pedwarn in the attribute handler is in
appropriate way of diagnosing an invalid use); they not have the
information about whether the attribute was a gnu:: one and can
legitimately accept a wider range of uses for the gnu:: attributes.

This patch ensures appropriate diagnostics for invalid uses of C2x
standard attributes on types, and so helps pave the way for the
subsequent removal of the bogus check in attribs.c, by adding a check
run in the front end before calling decl_attributes; this check
removes the attributes from the list after calling pedwarn to avoid
subsequent duplicate warnings.

Bootstrapped with no regressions for x86_64-pc-linux-gnu.

gcc/c:
	* c-decl.c (c_warn_type_attributes): New function.
	(groktypename, grokdeclarator, finish_declspecs): Call
	c_warn_type_attributes before applying attributes to types.
	* c-tree.h (c_warn_type_attributes): Declare.

gcc/testsuite:
	* gcc.dg/c2x-attr-deprecated-2.c, gcc.dg/c2x-attr-fallthrough-2.c,
	gcc.dg/c2x-attr-maybe_unused-2.c: Expect errors for invalid uses
	of standard attributes on types.  Add more tests of invalid uses
	on types.

From-SVN: r278471