[mlir] Add metadata to Diagnostic. (#99398)

Add metadata to Diagnostic.

Motivation: we have a use case where we want to do some filtering in our
customized Diagnostic Handler based on some cust

[mlir] Add metadata to Diagnostic. (#99398)

Add metadata to Diagnostic.

Motivation: we have a use case where we want to do some filtering in our
customized Diagnostic Handler based on some customized info that is not
`location` or `severity` or `diagnostic arguments` that are member
variables of `Diagnostic`. Specifically, we want to add a unique ID to
the `Diagnostic` for the handler to filter in a compiler pass that emits
errors in async tasks with multithreading and the diagnostic handling is
associated to the task.

This patch adds a field of `metadata` to `mlir::Diagnostics` as a
general solution. `metadata` is of type `SmallVector<DiagnosticArgument,
0>` to save memory size and reuse existing `DiagnosticArgument` for
metadata type.

show more ...