[mlir][IR] Add support for UnknownLoc to `verify-diagnostics` (#134421)
Diagnostics at unknown locations can now be verified with
`-verify-diagnostics`.
Example:
```
// expected-error@unknown {{something went wrong}}
```
Also clean up some MemRefToLLVM conversion tests that had to redirect
all errors to stdout in order to FileCheck them. All of those tests can
now be stored in a single `invalid.mlir`. That was not possible before.