compiler_types: mark __compiletime_assert failure as __noreturn

`__compiletime_assert` declares a fake `extern` function
which appears (to the compiler) to be called when the test fails.

Therefore, compilers may emit possibly-uninitialized warnings
in some cases, even if it will be an error anyway (for compilers
supporting the `error` attribute, e.g. GCC and Clang >= 14)
or a link failure (for those that do not, e.g. Clang < 14).

Annotating the fake function as `__noreturn` gives them
the information they need to avoid the warning,
e.g. see https://godbolt.org/z/x1v69jjYY.

Link: https://lore.kernel.org/llvm/202110100514.3h9CI4s0-lkp@intel.com/Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

include/linux/compiler_types.h

@@ -298,7 +298,13 @@ struct ftrace_likely_data {
 #ifdef __OPTIMIZE__
 # define __compiletime_assert(condition, msg, prefix, suffix)		\
 	do {								\
-		extern void prefix ## suffix(void) __compiletime_error(msg); \
+		/*							\
+		 * __noreturn is needed to give the compiler enough	\
+		 * information to avoid certain possibly-uninitialized	\
+		 * warnings (regardless of the build failing).		\
+		 */							\
+		__noreturn extern void prefix ## suffix(void)		\
+			__compiletime_error(msg);			\
 		if (!(condition))					\
 			prefix ## suffix();				\
 	} while (0)