fix Issue 21452 - isCallable erroneously returns false on function templates

[Bolpat]

This should be easy to review.

[dlang-bot]

Thanks for your pull request and interest in making D better, @Bolpat! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21452	normal	isCallable erroneously returns false on function templates

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 + phobos#7722"