preprocessor: Adjust C rules on UCNs for C23 [PR117162]

As noted in bug 117162, C23 changed some rules on UCNs to match C++
(this was a late change agreed in the resolution to CD2 comment
US-032, implementing changes from N3124), which we need to implement.

Allow UCNs below 0xa0 outside identifiers for C, with a
pedwarn-if-pedantic before C23 (and a warning with -Wc11-c23-compat)
except for the always-allowed cases of UCNs for $ @ `.  Also as part
of that change, do not allow \u0024 in identifiers as equivalent to $
for C23.

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

	PR c/117162

libcpp/
	* include/cpplib.h (struct cpp_options): Add low_ucns.
	* init.cc (struct lang_flags, lang_defaults): Add low_ucns.
	(cpp_set_lang): Set low_ucns
	* charset.cc (_cpp_valid_ucn): For C, allow UCNs below 0xa0
	outside identifiers, with a pedwarn if pedantic before C23 or a
	warning with -Wc11-c23-compat.  Do not allow \u0024 in identifiers
	for C23.

gcc/testsuite/
	* gcc.dg/cpp/c17-ucn-1.c, gcc.dg/cpp/c17-ucn-2.c,
	gcc.dg/cpp/c17-ucn-3.c, gcc.dg/cpp/c17-ucn-4.c,
	gcc.dg/cpp/c23-ucn-2.c, gcc.dg/cpp/c23-ucnid-2.c: New tests.
	* c-c++-common/cpp/delimited-escape-seq-3.c,
	c-c++-common/cpp/named-universal-char-escape-3.c,
	gcc.dg/cpp/c23-ucn-1.c, gcc.dg/cpp/c2y-delimited-escape-seq-3.c:
	Update expected messages
	* gcc.dg/cpp/ucs.c: Use -pedantic-errors.  Update expected
	messages.

libcpp/include/cpplib.h

@@ -557,6 +557,9 @@ struct cpp_options
   /* Nonzero for C++23 named universal character escape sequences.  */
   unsigned char named_uc_escape_seqs;
 
+  /* Nonzero for C++ and C23 UCNs for characters below 0xa0.  */
+  unsigned char low_ucns;
+
   /* Nonzero for C2Y 0o prefixed octal integer constants.  */
   unsigned char octal_constants;