Fix destructor return type in __cxa_init_primary_exception

llvm/llvm-project#65534 introduces
`__cxa_init_primary_exception` and uses this in libcxx. Like several
other methods like the below in `cxa_exception.cpp`,
https://github.com/emscripten-core/emscripten/blob/fbdd9249e939660cb0d20f00d6bc1897c2f3905e/system/lib/libcxxabi/src/cxa_exception.cpp#L264-L269
this new function takes a pointer to a destructor, and in Wasm
destructor returns a `this` pointer, which requires
`ifdef __USING_WASM_EXCEPTION__` on its signature.

And that it is also used in libcxx means we need to guard some code in
libcxx with `ifdef __USING_WASM_EXCEPTION__` for the signature
difference, and we need to build libcxx with
`-D__USING_WASM_EXCEPTIONS__` when Wasm EH is used.

Author: aheejin

system/lib/libcxx/include/__exception/exception_ptr.h

@@ -36,11 +36,13 @@ struct __cxa_exception;
 _LIBCPP_OVERRIDABLE_FUNC_VIS __cxa_exception* __cxa_init_primary_exception(
     void*,
     std::type_info*,
-    void(
 #  if defined(_WIN32)
-        __thiscall
+    void(__thiscall*)(void*)) throw();
+#  elif defined(__USING_WASM_EXCEPTIONS__)
+    void* (*)(void*)) throw();
+#  else
+    void (*)(void*)) throw();
 #  endif
-            *)(void*)) throw();
 }
 
 } // namespace __cxxabiv1
@@ -88,8 +90,15 @@ _LIBCPP_HIDE_FROM_ABI exception_ptr make_exception_ptr(_Ep __e) _NOEXCEPT {
   using _Ep2 = __decay_t<_Ep>;
 
   void* __ex = __cxxabiv1::__cxa_allocate_exception(sizeof(_Ep));
+#      ifdef __USING_WASM_EXCEPTIONS__
+  (void)__cxxabiv1::__cxa_init_primary_exception(__ex, const_cast<std::type_info*>(&typeid(_Ep)), [](void* __p) -> void* {
+#      else
   (void)__cxxabiv1::__cxa_init_primary_exception(__ex, const_cast<std::type_info*>(&typeid(_Ep)), [](void* __p) {
+#      endif
     std::__destroy_at(static_cast<_Ep2*>(__p));
+#      ifdef __USING_WASM_EXCEPTIONS__
+    return __p;
+#      endif
   });
 
   try {

system/lib/libcxxabi/include/cxxabi.h

@@ -48,7 +48,11 @@ extern _LIBCXXABI_FUNC_VIS void
 __cxa_free_exception(void *thrown_exception) throw();
 // This function is an LLVM extension, which mirrors the same extension in libsupc++ and libcxxrt
 extern _LIBCXXABI_FUNC_VIS __cxa_exception*
+#ifdef __USING_WASM_EXCEPTIONS__
+__cxa_init_primary_exception(void* object, std::type_info* tinfo, void*(_LIBCXXABI_DTOR_FUNC* dest)(void*)) throw();
+#else
 __cxa_init_primary_exception(void* object, std::type_info* tinfo, void(_LIBCXXABI_DTOR_FUNC* dest)(void*)) throw();
+#endif
 
 // 2.4.3 Throwing the Exception Object
 extern _LIBCXXABI_FUNC_VIS _LIBCXXABI_NORETURN void

system/lib/libcxxabi/src/cxa_exception.cpp

@@ -207,7 +207,12 @@ void __cxa_free_exception(void *thrown_object) throw() {
 }
 
 __cxa_exception* __cxa_init_primary_exception(void* object, std::type_info* tinfo,
+#ifdef __USING_WASM_EXCEPTIONS__
+// In Wasm, a destructor returns its argument
+                                              void *(_LIBCXXABI_DTOR_FUNC* dest)(void*)) throw() {
+#else
                                               void(_LIBCXXABI_DTOR_FUNC* dest)(void*)) throw() {
+#endif
   __cxa_exception* exception_header = cxa_exception_from_thrown_object(object);
   exception_header->referenceCount = 0;
   exception_header->unexpectedHandler = std::get_unexpected();

tools/system_libs.py

@@ -1619,6 +1619,12 @@ class libcxx(NoExceptLibrary, MTLibrary):
     'libdispatch.cpp',
   ]
 
+  def get_cflags(self):
+    cflags = super().get_cflags()
+    if self.eh_mode == Exceptions.WASM:
+      cflags.append('-D__USING_WASM_EXCEPTIONS__')
+    return cflags
+
 
 class libunwind(NoExceptLibrary, MTLibrary):
   name = 'libunwind'