[clang] [llvm] [WebAssembly] Implement an alternative translation for -wasm-enable-sjlj (PR #84137)

[Heejin Ahn via cfe-commits]

https://github.com/aheejin commented:

Nice simplification! Thank you for working on this.

Some minor nits about comments:
- You removed https://github.com/llvm/llvm-project/blob/43fc921795bd130a325c013d60f209b5c6128fc7/llvm/lib/Target/WebAssembly/WebAssemblyLowerEmscriptenEHSjLj.cpp#L213-L215 in https://github.com/emscripten-core/emscripten/pull/21502, so I think we should fix this explanation.
- There still seem to be mentions of `testSetjmp`, which should be changed to `__wasm_setjmp_test`.

https://github.com/llvm/llvm-project/pull/84137