preprocessor: Disable trigraphs for C2x

ISO C2x removes trigraphs, so disable them accordingly for -std=c2x
(they can still be enabled using -trigraphs).

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

libcpp/
	* init.cc (lang_defaults): Disable trigraphs for C2x.

gcc/testsuite/
	* gcc.dg/cpp/c11-trigraphs-1.c, gcc.dg/cpp/c2x-trigraphs-1.c,
	gcc.dg/cpp/c2x-trigraphs-2.c: New tests.