[libunwind] [libunwind] Move wasm definition of `__cpp_exception` (PR #195681)

[Sam Clegg via cfe-commits]

https://github.com/sbc100 commented:

Actually I think the fix is not going to very complex.   I should have it ready today (or at least this week).

So maybe this change is not needed?

When I suggested this workaround I was assuming it would be something you could apply downstream until the next LLVM release.   If the fix requires pulling in a new llvm version than I think we maybe we just wait for the LTO fix? 

https://github.com/llvm/llvm-project/pull/195681