bug#61481: CC Mode 5.35.2 (C/*l); _Generic unsupported

Package: cc-mode

2011 Standard C has a feature that works like `switch', except on the
type of an expression and with different syntax.

Today, I had to figure out how to port some code written with that in
mind to ANSI C, and surprisingly enough, found that CC Mode does not
support that feature at all.

Code making use of _Generic looks more or less like this:

{
  const char *typename = _Generic ((expr),
				   const char *:
				     "const char *",
				   int:
				     "int",
				   unsigned long:
				     "unsigned long"
				   default:
				     NULL);
}

where `typename' is set to an appropriate value based on the type of the
expression ``expr''.

CC Mode already works remarkably well, but it should be taught to indent
the cases separately from the values.

Emacs  : GNU Emacs 29.0.60 (build 1, x86_64-pc-linux-gnu)
 of 2022-12-24
Package: CC Mode 5.35.2 (C/*l)
Buffer Style: gnu
c-emacs-features: (pps-extended-state col-0-paren posix-char-classes gen-string-delim gen-comment-delim syntax-properties category-properties 1-bit)

current state:
==============
(setq
 c-basic-offset 2
 c-comment-only-line-offset '(0 . 0)
 c-indent-comment-alist '((anchored-comment column . 0) (end-block space . 1) (cpp-end-block space . 2))
 c-indent-comments-syntactically-p nil
 c-block-comment-prefix ""
 c-comment-prefix-regexp '((pike-mode . "//+!?\\|\\**") (awk-mode . "#+") (other . "//+\\|\\**"))
 c-doc-comment-style '((java-mode . javadoc) (pike-mode . autodoc) (c-mode . gtkdoc) (c++-mode . gtkdoc))
 c-cleanup-list '(scope-operator)
 c-hanging-braces-alist '((substatement-open before after) (arglist-cont-nonempty))
 c-hanging-colons-alist nil
 c-hanging-semi&comma-criteria '(c-semi&comma-inside-parenlist)
 c-backslash-column 48
 c-backslash-max-column 72
 c-special-indent-hook '(t c-gnu-impose-minimum)
 c-label-minimum-indentation 1
 c-offsets-alist '((inexpr-class . +)
		   (inexpr-statement . +)
		   (lambda-intro-cont . +)
		   (inlambda . 0)
		   (template-args-cont c-lineup-template-args +)
		   (incomposition . +)
		   (inmodule . +)
		   (innamespace . +)
		   (inextern-lang . +)
		   (composition-close . 0)
		   (module-close . 0)
		   (namespace-close . 0)
		   (extern-lang-close . 0)
		   (composition-open . 0)
		   (module-open . 0)
		   (namespace-open . 0)
		   (extern-lang-open . 0)
		   (objc-method-call-cont c-lineup-ObjC-method-call-colons c-lineup-ObjC-method-call +)
		   (objc-method-args-cont . c-lineup-ObjC-method-args)
		   (objc-method-intro . [0])
		   (friend . 0)
		   (cpp-define-intro c-lineup-cpp-define +)
		   (cpp-macro-cont . +)
		   (cpp-macro . [0])
		   (inclass . +)
		   (stream-op . c-lineup-streamop)
		   (arglist-cont-nonempty c-lineup-gcc-asm-reg c-lineup-arglist)
		   (arglist-cont c-lineup-gcc-asm-reg 0)
		   (comment-intro c-lineup-knr-region-comment c-lineup-comment)
		   (catch-clause . 0)
		   (else-clause . 0)
		   (do-while-closure . 0)
		   (access-label . -)
		   (case-label . 0)
		   (substatement . +)
		   (statement-case-intro . +)
		   (statement . 0)
		   (brace-entry-open . 0)
		   (brace-list-entry . 0)
		   (brace-list-close . 0)
		   (block-close . 0)
		   (block-open . 0)
		   (inher-cont . c-lineup-multi-inher)
		   (inher-intro . +)
		   (member-init-cont . c-lineup-multi-inher)
		   (member-init-intro . +)
		   (annotation-var-cont . +)
		   (annotation-top-cont . 0)
		   (topmost-intro . 0)
		   (knr-argdecl . 0)
		   (func-decl-cont . +)
		   (inline-close . 0)
		   (class-close . 0)
		   (class-open . 0)
		   (defun-block-intro . +)
		   (defun-close . 0)
		   (defun-open . 0)
		   (c . c-lineup-C-comments)
		   (string . c-lineup-dont-change)
		   (topmost-intro-cont first c-lineup-topmost-intro-cont c-lineup-gnu-DEFUN-intro-cont)
		   (brace-list-intro first c-lineup-2nd-brace-entry-in-arglist c-lineup-class-decl-init-+ +)
		   (brace-list-open . +)
		   (inline-open . 0)
		   (arglist-close . c-lineup-arglist)
		   (arglist-intro . c-lineup-arglist-intro-after-paren)
		   (statement-cont . +)
		   (statement-case-open . +)
		   (label . 0)
		   (substatement-label . 0)
		   (substatement-open . +)
		   (knr-argdecl-intro . 5)
		   (statement-block-intro . +)
		   )
 c-buffer-is-cc-mode 'c-mode
 c-tab-always-indent t
 c-syntactic-indentation t
 c-syntactic-indentation-in-macros t
 c-ignore-auto-fill '(string cpp code)
 c-auto-align-backslashes t
 c-backspace-function 'backward-delete-char-untabify
 c-delete-function 'delete-char
 c-electric-pound-behavior nil
 c-default-style '((java-mode . "java") (awk-mode . "awk") (other . "gnu"))
 c-enable-xemacs-performance-kludge-p nil
 c-old-style-variable-behavior nil
 defun-prompt-regexp nil
 tab-width 8
 comment-column 32
 parse-sexp-ignore-comments t
 parse-sexp-lookup-properties t
 auto-fill-function nil
 comment-multi-line t
 comment-start-skip "\\(?://+\\|/\\*+\\)\\s *"
 fill-prefix nil
 fill-column 70
 paragraph-start "[ 	]*\\(//+\\|\\**\\)[ 	]*$\\|^\f"
 adaptive-fill-mode t
 adaptive-fill-regexp "[ 	]*\\(//+\\|\\**\\)[ 	]*\\([ 	]*\\([-–!|#%;>*·•‣⁃◦]+[ 	]*\\)*\\)"
 )