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.