c++: Pedwarn on [[]]; at class scope [PR110345]HEADtrunkmaster

For C++ 26 P2552R3 I went through all the spots (except modules) where
attribute-specifier-seq appears in the grammar and tried to construct
a testcase in all those spots, for now for [[deprecated]] attribute.

The fourth issue is that we just emit (when enabled) -Wextra-semi warning
not just for lone semicolon at class scope (correct), but also for
[[]]; or [[whatever]]; there too.
While just semicolon is valid in C++11 and newer,
https://eel.is/c++draft/class.mem#nt:member-declaration
allows empty-declaration, unlike namespace scope or block scope
something like attribute-declaration or empty statement with attributes
applied for it aren't supported.
While syntactically it matches
attribute-specifier-seq [opt] decl-specifier-seq [opt] member-declarator-list [opt] ;
with the latter two omitted, there is
https://eel.is/c++draft/class.mem#general-3
which says that is not valid.

So, the following patch emits a pedwarn in that case.

2024-08-16  Jakub Jelinek  <jakub@redhat.com>

	PR c++/110345
	* parser.cc (cp_parser_member_declaration): Call maybe_warn_extra_semi
	only if it is empty-declaration, if there are some tokens like
	attribute, pedwarn that the declaration doesn't declare anything.

	* g++.dg/cpp0x/gen-attrs-84.C: New test.

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