c++: add missing -Wc++??-extensions checks

The pedwarns for each of these features should be silenced by
the appropriate -Wno-c++??-extensions.

The handle_pragma_diagnostic_impl change is necessary so that we handle
-Wc++23-extensions early so it's available to interpret_float while lexing.

gcc/c-family/ChangeLog:

	* c-pragma.cc (handle_pragma_diagnostic_impl): Also handle
	-Wc++23-extensions early.
	* c-lex.cc (interpret_float): Use -Wc++23-extensions for extended
	floating point literal pedwarn.

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_simple_type_specifier): Use
	-Wc++20-extensions for auto parameter pedwarn.
	* pt.cc (do_decl_instantiation, do_type_instantiation): Use
	-Wc++11-extensions for 'extern template'.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/extern_template-7.C: New test.
	* g++.dg/cpp23/ext-floating19.C: New test.
	* g++.dg/cpp2a/abbrev-fn1.C: New test.