Fix lto0.test_exceptions_allowed_uncaught failure

[sbc100]

This test started failing after #16627. Prior to that change the exceptions destructors were called via the following code in JS:

   // In Wasm, destructors return 'this' as in ARM
   {{{ makeDynCall('pp', 'destructor') }}}(info.excPtr);

For some reason the LTO build produces the following for for the call
to exception_header->exceptionDestructor(thrown_object) in
__cxa_decrement_exception_refcount:

  call_indirect 0 (type 0)                                               

Where as the normal non-LTO build produces:

  call 13 <invoke_ii>                                                    

Because invoke_ii goes via JS it uses the sloppy type checking and
doesn't trap, but call_indirect has strict type checking and so
does trap.

[sbc100]

I figured out the problem. See the new description.

[RReverser]

Thanks, I see now. So technically signature was incorrect even before the PR, but because it was used only from JS, it didn't matter?

[aheejin]

I remember adding this years ago... I guess it was somehow lost during library updates. https://github.com/aheejin/emscripten/blob/d57db5bea1719319a680699c50b91fa3d88fa0ec/system/lib/libcxxabi/src/cxa_exception.hpp#L41-L46

By the way, wouldn't it be better to add ifdef as in the old code, in case when we try to upstream our changes?

[sbc100]

This entire block is already within #ifdef __USING_EMSCRIPTEN_EXCEPTIONS__.

BTW, is there some way we can undef the destructors return 'this' as in ARM decision, since it makes all these patches needed? Is there some reason we need this behaviour?

[dschuff]

Maybe? AFAIK @sunfishcode added that way back at the beginning, as a performance optimization. I don't know how much impact it actually has.