preprocessor: Enable digit separators for C2X

C2X adds digit separators, as in C++.  Enable them accordingly in
libcpp and c-lex.c.  Some basic tests are added that digit separators
behave as expected for C2X and are properly disabled for C11; further
test coverage is included in the existing g++.dg/cpp1y/digit-sep*.C
tests.

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

gcc/c-family/
	* c-lex.c (interpret_float): Handle digit separators for C2X.

libcpp/
	* init.c (lang_defaults): Enable digit separators for GNUC2X and
	STDC2X.

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