Fix issue 20039: when checking for an existing template instance, tolerate expressions without types.

[FeepingCreature]

No description provided.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20039	normal	ICE from double template instantiation with getMember of overload of class and template function

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 "stable + dmd#10159"

[thewilsonator]

This ICE should probably go to stable.