[llvm] [ADT] Allow SmallVector move construction without move assignment (PR #219934)

[lonely eagle via llvm-commits]

linuxlonelyeagle wrote:

By the way, this piece of code is running pretty well now. Thanks a lot @joker-eph  and I learned something.
```
/// Emit a remark about this operation for each message, reporting up to
/// any diagnostic handlers that may be listening.
SmallVector<InFlightDiagnostic>
Operation::emitRemark(const ArrayRef<Twine> messages) {
  SmallVector<InFlightDiagnostic> diags;
  for (size_t i = 0, e = messages.size(); i < e; ++i) {
    InFlightDiagnostic diag = mlir::emitRemark(getLoc(), messages[i]);
    if (i == e - 1 && getContext()->shouldPrintOpOnDiagnostic())
      diag.attachNote(getLoc()) << "see current operation: " << *this;
    diags.push_back(std::move(diag));
  }
  return diags;
}
```

https://github.com/llvm/llvm-project/pull/219934