[EH] Fix inconsistency of in/decrementing refcounts

[aheejin]

This fixes inconsistency of incrementing/decrementing refcounts between Wasm EH and Emscripten EH.

Previously, in Emscripten EH, we incremented the refcount in __cxa_begin_catch, while Wasm EH incremented it in __cxa_throw. This PR moves the incrementing call from __cxa_begin_catch to __cxa_throw as well. This also increments the refcount in __cxa_rethrow.

These incrementing calls are guarded with
+#if !DISABLE_EXCEPTION_CATCHING, because without that, std::terminate will run:

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

Lines 25 to 35 in d125179

	void
	__cxa_increment_exception_refcount(void *thrown_object) throw() {
	if (thrown_object != nullptr)
	std::terminate();
	}
	void
	__cxa_decrement_exception_refcount(void *thrown_object) throw() {
	if (thrown_object != nullptr)
	std::terminate();
	}

Fixes #17115.

[kleisauke]

Great! It looks like these mentions can also be removed from the docs now:
https://github.com/emscripten-core/emscripten/blob/d1251798144df813c52934768964a1223504c440/src/settings.js#L764C45-L766C65
https://github.com/emscripten-core/emscripten/blob/d1251798144df813c52934768964a1223504c440/site/source/docs/porting/exceptions.rst?plain=1#L159C60-L167C52

emscripten/tools/link.py

Lines 1837 to 1838 in d125179

	# What you need to do is different depending on the kind of EH you use
	# (https://github.com/emscripten-core/emscripten/issues/17115).

[aheejin]

Great! It looks like these mentions can also be removed from the docs now: https://github.com/emscripten-core/emscripten/blob/d1251798144df813c52934768964a1223504c440/src/settings.js#L764C45-L766C65 https://github.com/emscripten-core/emscripten/blob/d1251798144df813c52934768964a1223504c440/site/source/docs/porting/exceptions.rst?plain=1#L159C60-L167C52

emscripten/tools/link.py

Lines 1837 to 1838 in d125179

	# What you need to do is different depending on the kind of EH you use
	# (https://github.com/emscripten-core/emscripten/issues/17115).

Thanks! Removed: 9fee482

[aheejin]

This fails LTO tests due to llvm/llvm-project#173235. Before, __cxa_increment_exception_refcount was a dependency for both __cxa_begin_catch and __cxa_current_primary_exception, and __cxa_begin_catch was (in the default Emscripten EH throwing-only mode) a JS function that was not used, so __cxa_current_primary_exception was preserved and exported anyway. But now __cxa_increment_exception_refcount is only dependent on __cxa_current_primary_exception, which is a JS function but in the throwing-only mode it is also defined in cxa_noexception.cpp, LTO tests fail. (See llvm/llvm-project#173235 for more explanation)

We have two choices:

1. Wait until [lld][WebAssembly] Preserve LTO stub deps for bitcode symbols llvm/llvm-project#173235 lands
2. Guard every usage of __cxa_in/decrement_exception_refcount with #if !DISABLE_EXCEPTION_CATCHING (Now I guarded only the ones in the function that can be used in throwing-only mode)

Maybe 2 is better for safety anyway.