[EH] Fix __get_exception_message for multiple virtual inheritance

[aheejin]

This fixes __get_exception_message so it works with multiple virtual inheritance. #24008 tried to fix this, but the test case there didn't virtual in the line where the inheritance happens.

In case of dependent exceptions (= the exceptions thrown by std::rethrow_exception), we should not even dereference thrown_object, because it doesn't contain anything. #24008 ran can_catch on the currect primary exception but before that it called can_catch on the dependent exception first, which caused the memory error. #24008's test case was fine because it didn't have virtual, we didn't have to dereference thrown_object (adjustedPtr in this method) directly:

emscripten/system/lib/libcxxabi/src/private_typeinfo.cpp

Lines 572 to 579 in b2e19a3

	if (info->have_object) {
	/* We have an object to inspect, we can look through its vtables to
	find the layout. */
	offset_to_base = __offset_flags >> __offset_shift;
	if (is_virtual) {
	const char* vtable = strip_vtable(*static_cast<const char* const*>(adjustedPtr));
	offset_to_base = update_offset_to_base(vtable, offset_to_base);
	}

This computes the primary exception pointer first and then call can_catch.

Fixes #26771.

[sbc100]

Do we really need test_multi_inheritance_exception_message and test_multi_inheritance_exception_message2 both? Or can we adapt the first one maybe?

Otherwise lgtm

[aheejin]

We can probably just delete the first one I think. Multiple inheritance seems almost always paired with virtual anyway.