c++: Implement C++23 P0330 - Literal Suffixes for ptrdiff_t and size_t.

Integer literal suffixes for signed size ('z') and unsigned size
(some permutation od 'zu') are provided as a language addition.

gcc/c-family/ChangeLog:

	* c-cppbuiltin.c (c_cpp_builtins): Define __cpp_size_t_suffix.
	* c-lex.c (interpret_integer): Set node type for size literal.

libcpp/ChangeLog:

	* expr.c (interpret_int_suffix): Detect 'z' integer suffix.
	(cpp_classify_number): Compat warning for use of 'z' suffix.
	* include/cpplib.h (struct cpp_options): New flag.
	(enum cpp_warning_reason): New flag.
	(CPP_N_USERDEF): Comment C++0x -> C++11.
	(CPP_N_SIZE_T): New flag for cpp_classify_number.
	* init.c (cpp_set_lang): Initialize new flag.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/udlit-shadow-neg.C: Test for 'z' and 'zu' shadowing.
	* g++.dg/cpp23/feat-cxx2b.C: New test.
	* g++.dg/cpp23/size_t-literals.C: New test.
	* g++.dg/warn/Wsize_t-literals.C: New test.