if no output is specified to wasm-opt, warn that we are emitting nothing (#1908)

A user that just does

```
wasm-opt input.wasm -O
```
may assume that the input file should have been optimized. But without `-o` we don't emit any output.

Often you may not want any output, like if you just want to run a pass like `--metrics`. But for most users wasm-opt is probably going to be used as an optimizer of files. So this PR suggests we emit a warning in that case.

For comparison, `llvm-opt` would print to the console, but it avoids printing a binary there so it issues a warning. Instead of this warning, perhaps we should do the same? That would also not be confusing.

Closes #1907

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