c++: Parse and ignore attributes on base specifiers [PR110345]

c++: Parse and ignore attributes on base specifiers [PR110345]

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.

This is the third issue I found.

https://eel.is/c++draft/class.derived#general-1 has attribute-specifier-seq
at the start of base-specifier.  The following patch parses it there and
warns about those.

2024-08-20  Jakub Jelinek  <jakub@redhat.com>

	PR c++/110345
	* parser.cc (cp_parser_base_specifier): Parse standard attributes
	at the start and emit a warning if there are any non-ignored ones.

	* g++.dg/cpp0x/gen-attrs-83.C: New test.