libcpp: adjust pedwarn handling

Using cpp_pedwarning (CPP_W_PEDANTIC instead of if (CPP_PEDANTIC cpp_error
lets users suppress these diagnostics with
 #pragma GCC diagnostic ignored "-Wpedantic".

This patch changes all instances of the cpp_error (CPP_DL_PEDWARN to
cpp_pedwarning.  In cases where the extension appears in a later C++
revision, we now condition the warning on the relevant -Wc++??-extensions
flag instead of -Wpedantic; in such cases often the if (CPP_PEDANTIC) check
is retained to preserve the default non-warning behavior.

I didn't attempt to adjust the warning flags for the C compiler, since it
seems to follow a different system than C++.

The CPP_PEDANTIC check is also kept in _cpp_lex_direct to avoid an ICE in
the self-tests from cb.diagnostics not being initialized.

While working on testcases for these changes I noticed that the c-c++-common
tests are not run with -pedantic-errors by default like the gcc.dg and
g++.dg directories are.  And if I specify -pedantic-errors with dg-options,
the default -std= changes from c++?? to gnu++??, which interferes with some
other pedwarns.  So two of the tests are C++-only.

libcpp/ChangeLog:

	* include/cpplib.h (enum cpp_warning_reason): Add
	CPP_W_CXX{14,17,20,23}_EXTENSIONS.
	* charset.cc (_cpp_valid_ucn, convert_hex, convert_oct)
	(convert_escape, narrow_str_to_charconst): Use cpp_pedwarning
	instead of cpp_error for pedwarns.
	* directives.cc (directive_diagnostics, _cpp_handle_directive)
	(do_line, do_elif): Likewise.
	* expr.cc (cpp_classify_number, eval_token): Likewise.
	* lex.cc (skip_whitespace, maybe_va_opt_error)
	(_cpp_lex_direct): Likewise.
	* macro.cc (_cpp_arguments_ok): Likewise.
	(replace_args): Use -Wvariadic-macros for pedwarn about
	empty macro arguments.

gcc/c-family/ChangeLog:

	* c.opt: Add CppReason for Wc++{14,17,20,23}-extensions.
	* c-pragma.cc (handle_pragma_diagnostic_impl): Don't check
	OPT_Wc__23_extensions.

gcc/testsuite/ChangeLog:

	* c-c++-common/pragma-diag-17.c: New test.
	* g++.dg/cpp0x/va-opt1.C: New test.
	* g++.dg/cpp23/named-universal-char-escape3.C: New test.