[libc++][NFC] Replace _LIBCPP_NORETURN and TEST_NORETURN with [[noreturn]] (#80455)
`[[__noreturn__]]` is now always available, so we can simply use the
attribute directly instead of through a macro.
diff --git a/libcxx/src/stdexcept.cpp b/libcxx/src/stdexcept.cpp
index bc25c0f..134d28e 100644
--- a/libcxx/src/stdexcept.cpp
+++ b/libcxx/src/stdexcept.cpp
@@ -19,7 +19,7 @@
_LIBCPP_BEGIN_NAMESPACE_STD
-_LIBCPP_NORETURN void __throw_runtime_error(const char* msg) {
+void __throw_runtime_error(const char* msg) {
#ifndef _LIBCPP_HAS_NO_EXCEPTIONS
throw runtime_error(msg);
#else
