c: Handle scoped attributes in __has*attribute and scoped attribute parsing changes...

c: Handle scoped attributes in __has*attribute and scoped attribute parsing changes in -std=c11 etc. modes [PR114007]

We aren't able to parse __has_attribute (vendor::attr) (and __has_c_attribute
and __has_cpp_attribute) in strict C < C23 modes.  While in -std=gnu* modes
or in -std=c23 there is CPP_SCOPE token, in -std=c* (except for -std=c23)
there are is just a pair of CPP_COLON tokens.
The c-lex.cc hunk adds support for that, but always returns 0 in that case
unlike the GCC 14+ version.

2024-02-22  Jakub Jelinek  <jakub@redhat.com>

	PR c/114007
gcc/c-family/
	* c-lex.cc (c_common_has_attribute): Parse 2 CPP_COLONs with
	the first one with COLON_SCOPE flag the same as CPP_SCOPE but
	ensure 0 is returned then.
gcc/testsuite/
	* gcc.dg/c23-attr-syntax-8.c: New test.
libcpp/
	* include/cpplib.h (COLON_SCOPE): Define to PURE_ZERO.
	* lex.cc (_cpp_lex_direct): When lexing CPP_COLON with another
	colon after it, if !CPP_OPTION (pfile, scope) set COLON_SCOPE
	flag on the first CPP_COLON token.

(cherry picked from commit 37127ed975e09813eaa2d1cf1062055fce45dd16)