[mlir] "expected" diagnostics are checked even when `-verify-diagnostics` flag is not used

Commit: 2e2c316fd5d7e8cff94703992928d5882d00478e

Input:
```mlir
// expected-error @unknown {{an error that is never produced}}
module {}
```

Command:
`build/bin/mlir-opt input.mlir`

Actual output:
```
module {
}

:0: error: expected error "an error that is never produced" was not produced
```

Expected output:
```
module {
}
```

Relevant code:
https://github.com/llvm/llvm-project/blob/847fb5cfde68fdea490171bc24087ebb0cd90738/mlir/lib/Tools/mlir-opt/MlirOptMain.cpp#L777-L796

The `SourceMgrDiagnosticVerifierHandler` is created unconditionally rather than guarded by `config.shouldVerifyDiagnostics()` and its destructor calls `verify()` which results in unsatisfied "expected" declarations being reported even when `config.shouldVerifyDiagnostics()` is false.

Start in mlir/lib/Tools/mlir-opt/MlirOptMain.cpp around lines 777-796, where the diagnostic verifier handler is created, and reproduce the issue with build/bin/mlir-opt input.mlir using the supplied MLIR input. Repeat with -verify-diagnostics. Done when the unsatisfied expected diagnostic is reported only when that flag is enabled.

Written by the indexing model from the issue text.