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

[Alex Crichton via cfe-commits]

alexcrichton wrote:

I can see how the current state of the world is resulting in this PR, but at least personally I continue to feel that a warning should not be emitted if no flags are passed and `wasm-opt` is not found. Part of it I've [mentioned above](https://github.com/llvm/llvm-project/pull/100321#issuecomment-2248296424) which I can summarize as: a warning would mean that the default installation of wasi-sdk would generate warnings for projects by default. Personally I don't think this is a great situation to be in.

The other part of it is that personally I often specifically don't want `wasm-opt` to run. I keep it out of my `PATH` as a result and it runs the risk of making debugging harder by adding a tool in the toolchain to chase down when something goes wrong.

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