[EH] Remove -wasm-enable-eh LLVM flag

[aheejin]

Previously in LLVM, we had -mllvm -enable-emscripten-cxx-exceptions to enable Emscriten EH, and -mllvm -wasm-enable-eh to enable Wasm EH. These were separate Wasm backend flags.

Also there is a separate -exception-model= LLVM flag used for all LLVM targets. We used wasm for Wasm EH and none for Emscripten EH. This was a common LLVM flag which we didn't directly use but fed LLVM because it was a common flag.

Recently a series of Clang+LLVM EH flag refactoring was done, and now LLVM is using -exception-model to control all EH modes. So Emscripten EH will be enabled by -exception-model=emscripten, and Wasm EH by -exception-model=wasm, and we don't need the Wasm-specific LLVM flags. -enable-emscripten-cxx-exceptions was removed in #27704. This removes -wasm-enable-eh.

This has to land before llvm/llvm-project#224313. This also temporarily disable EH LTO tests, which we will reenable after llvm/llvm-project#224313 lands.

[dschuff]

Do we need any of the pieces of llvm/llvm-project#224313 to land for this to work, for example the change to CodeGenPassBuilder.cpp and WebAssemblyTargetMachine.h?

[aheejin]

We have to hand this before llvm/llvm-project#224313 lands, because it removes -wasm-enable-eh, and without this PR Emscripten will still feed -wasm-enable-eh. But it turns out we can't land this first either because without llvm/llvm-project#224313 landed first we can't remove -wasm-enable-eh here because LTO EH tests are failing.

I think we should temporarily disable LTO tests here, and land llvm/llvm-project#224313, and re-enable them.