Fix Issue 20419 - is(missing == module/package) results in unknown id…

[MoonlightSentinel]

…entifier.

is(missing == module) and is(missing == package) raised an error for missing identifiers, violating the language specification wich only requires the identifier to be syntactically valid.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20419	normal	is(missing == module/package) results in unknown identifier

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + dmd#10621"

[Geod24]

Shouldn't this have gone to stable ?

[MoonlightSentinel]

Shouldn't this have gone to stable ?

Why, isn't stable reserved for regressions?
(IMO this does not qualify as a regression fix because these traits were introduced with this flaw)

[Geod24]

It's also used for non-disruptive bugfixes where there is a clear benefit to the bugfix.
IMO this PR is a textbook example of that :)

[MoonlightSentinel]

Okay, I'll try to remember that for my future PRs