preprocessor: Enable __VA_OPT__ for C2x

C2x supports __VA_OPT__, so adjust libcpp not to pedwarn for uses of
it (or of not passing any variable arguments to a variable-arguments
macro) in standard C2x mode.

I didn't try to duplicate existing tests for the details of the
feature, just verified -pedantic-errors handling is as expected.  And
there's a reasonable argument (bug 98859) that __VA_OPT__ shouldn't be
diagnosed in older standard modes at all (as opposed to not passing
any variable arguments to a variable-arguments macro, for which older
versions of the C standard require a diagnostic as a constraint
violation); that argument applies to C as much as to C++, but I
haven't made any changes in that regard.

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

libcpp/
	* init.cc (lang_defaults): Enable va_opt for STDC2X.
	* lex.cc (maybe_va_opt_error): Adjust diagnostic message for C.
	* macro.cc (_cpp_arguments_ok): Update comment.

gcc/testsuite/
	* gcc.dg/cpp/c11-vararg-1.c, gcc.dg/cpp/c2x-va-opt-1.c: New tests.