[clang] Add wasm-opt warning (PR #100321)

[Alex Crichton via cfe-commits]

alexcrichton wrote:

Personally I don't find it to be a great user experience when a tool warns by default in default configration. This means that any user using wasi-sdk, for example, would by default with no other action taken receive warnings. That's annoying and misleading to end users because `wasm-opt` isn't required for a successful compilation. "Just add another flag" does indeed solve the problem but it's annoying to have to configure that and IMO it should not be required to pass extra flags to suppress warnings in default configurations. 

https://github.com/llvm/llvm-project/pull/100321