c: C2x binary constants

c: C2x binary constants

C2x adds binary integer constants (approved at the last WG14 meeting,
though not yet added to the working draft in git).  Configure libcpp
to consider these a standard feature in C2x mode, with appropriate
updates to diagnostics including support for diagnosing them with
-std=c2x -Wc11-c2x-compat.

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

gcc/testsuite/
2020-11-13  Joseph Myers  <joseph@codesourcery.com>

	* gcc.dg/binary-constants-2.c, gcc.dg/binary-constants-3.c,
	gcc.dg/system-binary-constants-1.c: Update expected diagnostics.
	* gcc.dg/c11-binary-constants-1.c,
	gcc.dg/c11-binary-constants-2.c, gcc.dg/c2x-binary-constants-1.c,
	gcc.dg/c2x-binary-constants-2.c, gcc.dg/c2x-binary-constants-3.c:
	New tests.

libcpp/
2020-11-13  Joseph Myers  <joseph@codesourcery.com>

	* expr.c (cpp_classify_number): Update diagnostic for binary
	constants for C.  Also diagnose binary constants for
	-Wc11-c2x-compat.
	* init.c (lang_defaults): Enable binary constants for GNUC2X and
	STDC2X.