fix issue 20915 - exclude version and debug identifier from __traits(allMembers) results

[ghost]

This is the right fix this time, after clarification from @WalterBright on the semantics.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20915	major	__traits(allMembers) results include custom version identifiers, which is unusable

Testing this PR locally

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

dub run digger -- build "master + dmd#11260"

[Geod24]

Why not use either Condition or DVCondition ?

[ghost]

Using VersionSymbol and DebugSymbol there's no need to test the identifier in the table of identifiers attached to the module. The code to cast a result to a symbol is already there, just a little isStuff isThing and it's good. That's a completly straightforward way of filtering out the problematic things.

[WalterBright]

It took me a moment to figure out why the version and debug ids were even showing up in allMembers. Turns out it was inadvertent and they should be excluded. This PR looks good.