tfdiags: Sort order for diagnostics
tfdiags: Sort order for diagnostics

Because we gather together diagnostics from many different parts of the
codebase, the list often ends up being in a non-ideal order. Here we
define a partial ordering for diagnostics that should hopefully make them
easier to scan when many are present, by grouping together diagnostics
that are of the same severity and belong to the same file.

We use sort.Stable here because we have a partial order and so we need
to make sure that diagnostics that do not have a relative ordering will
remain in their original order.

This sorting is applied just in time before rendering the diagnostics
in command.Meta.showDiagnostics.

hashicorp/terraform-plugin-sdk,hashicorp/terraform-plugin-sdk