[verify] Improve the error messages with multiple active prefixes (#126068)

Multiple improvements to make the messages more concrete, actionable and
less confusing when multiple prefixes are used in `-verify=`. The common
theme among these was that prior to the patch all error messages would
use the alphabetically first prefix, even if the error was associated
with a different one.

- Mention the actual expected but unseen directive: Prior to this change
when reporting expected but unseen directive, the alphabetically first
one would be used to report the error even if that's not the one present
in the source. Reword the diagnostic if multiple prefixes are active and
include the real spelling of the expected directive for each expected
but not seen line in the output.

- Reword the seen but not expected error message if multiple directives
are active to avoid having to pick an arbitrary (the first) prefix for
it.

- Include the full spelling of the directive when reporting a directive
following the no-diagnostics directive. For example "'foo-error'
directive cannot follow 'foo-no-diagnostics' directive"

- Use the first appearing `-no-diagnostics` directive, in the above
message instead of the first one alphabetically.

The new wording
> diagnostics with '(error|warning|remark|note)' severity seen but not
expected

instead of

> '<prefix>-(error|warning|remark|note)' diagnostics seen but not
expected

is only used when multiple prefixes are present, the error messages stay
the same with a single prefix only.