C++23 support

[guwirth]

Support latest C++ standard C++23:

• Working Draft, Standard for Programming Language C++ 2023/n4944
• cppreference/C++23

C++23 feature	Paper(s)	cxx plugin	cxx version	pull request
Literal suffix for (signed) size_t	P0330R8	no	todo
Make () more optional for lambdas	P1102R2	no	todo
if consteval	P1938R3	no	todo
Removing Garbage Collection Support	P2186R2	no		2)
Narrowing contextual conversions in static_assert and constexpr if	P1401R5	no		2)
Trimming whitespaces before line splicing	P2223R2	no	todo
Make declaration order layout mandated	P1847R4	no		2)
Removing mixed wide string literal concatenation	P2201R1	no	todo	3)
Explicit object parameter (deducing this)	P0847R7	no	todo
auto(x) and auto{x}	P0849R8	no	todo
Change scope of lambda trailing-return-type	P2036R3	no		2)
#elifdef and #elifndef	P2334R1	YES	v2.1.0	#2411
Non-literal variables(and labels and gotos) in constexpr functions	P2242R3	no	todo
Consistent character literal encoding	P2316R2	no	todo
Character sets and encodings	P2314R4	no	todo
Extend init-statement (of for loop) to allow alias-declaration	P2360R0	no	todo
Multidimensional subscript operator	P2128R6	no	todo
Attributes on lambdas	P2173R1	no	todo
#warning	P2437R1	no	v2.0	1)
Remove non-encodable wide character literals and multicharacter wide character literals	P2362R3	no	todo
Labels at the end of compound statements	P2324R2	no	todo
Delimited escape sequences	P2290R3	no	todo
Named universal character escapes	P2071R2	no	todo
Relaxing some constexpr restrictions	P2448R2	no		2)
Simpler implicit move	P2266R3	no		2)
static operator()	P1169R4	no	todo
Requirements for optional extended floating-point types	P1467R9	no	todo
Class template argument deduction from inherited constructors	P2582R1	no		2)
Attribute [[assume]]	P1774R8	no	todo
Support for UTF-8 as a portable source file encoding	P2295R6	no	v2.0.2	#2143 1)
static operator[]	P2589R0	no	todo	3)
Permitting static constexpr variables in constexpr functions=	P2647R1	no	todo	3)
Extending the lifetime of temporaries in range-based for loop initializer	P2718R0	no		2)
DR20: Meaningful exports	P2615R0	no	todo
DR20: consteval needs to propagate up	P2564R0	no		2)
DR: C++ Identifier Syntax using Unicode Standard Annex 31	P1949R7	no	todo
DR11: Allow duplicate attributes	P2156R1	no	todo	3)
DR20: Adjusting the value of feature-test macro __cpp_concepts	P2493R0	no		2)
DR98: Relax requirements on wchar_t to match existing practices	P2460R2	no		2)
DR: Using unknown pointers and references in constant expressions	P2280R4	no		2)
DR20: The Equality Operator You Are Looking For	P2468R2	no		2)
DR20: char8_t Compatibility and Portability Fix	P2513R4	no		2)
DR: Clarify reportingof diagnostic directives and allow static_assert of non-value-dependent expressions in a template context	CWG2518	no		2)

1) already supported in previous versions
2) does not lead to a change in grammar
3) test of the current behavior necessary