libcxx: update to LLVM 18

release/18.x branch, commit 78b99c73ee4b96fe9ce0e294d4632326afb2db42

This adds the flag `-D_LIBCPP_HARDENING_MODE` which is determined based
on the Zig optimization mode.

This commit also fixes libunwind, libcxx, and libcxxabi to properly
report sub compilation errors.

lib/libcxx/src/stdexcept.cpp

@@ -6,12 +6,25 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include <__verbose_abort>
 #include <new>
 #include <stdexcept>
 #include <string>
 
 #ifdef _LIBCPP_ABI_VCRUNTIME
-#include "support/runtime/stdexcept_vcruntime.ipp"
+#  include "support/runtime/stdexcept_vcruntime.ipp"
 #else
-#include "support/runtime/stdexcept_default.ipp"
+#  include "support/runtime/stdexcept_default.ipp"
 #endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+_LIBCPP_NORETURN void __throw_runtime_error(const char* msg) {
+#ifndef _LIBCPP_HAS_NO_EXCEPTIONS
+  throw runtime_error(msg);
+#else
+  _LIBCPP_VERBOSE_ABORT("runtime_error was thrown in -fno-exceptions mode with message \"%s\"", msg);
+#endif
+}
+
+_LIBCPP_END_NAMESPACE_STD