[clang] [WebAssembly] Define __USING_WASM_EXCEPTIONS__ for -fwasm-exceptions (PR #92604)

[Sam Clegg via cfe-commits]

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@@ -1006,6 +1006,8 @@ static void InitializePredefinedMacros(const TargetInfo &TI,
   else if (LangOpts.hasDWARFExceptions() &&
            (TI.getTriple().isThumb() || TI.getTriple().isARM()))
     Builder.defineMacro("__ARM_DWARF_EH__");
+  else if (LangOpts.hasWasmExceptions() && TI.getTriple().isWasm())
+    Builder.defineMacro("__USING_WASM_EXCEPTIONS__");
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sbc100 wrote:

I don't think we should remove the define here, I just want us to consider carefully the name of the define if we are going to have the compiler generate it.

The removal I was referring to was the `-D...` flags used in emscripten and wasi-sdk, which will be redundant once this change land (and they convert to using the new macro).

https://github.com/llvm/llvm-project/pull/92604