When registering Ident use, consider the original span for synthetics

[natsukagami]

Fix #19970

• Consider the original span for synthetics for registering Ident use
• Clarify that tryFindSynthetic is for Apply | TypeApply tree
• Update tests

[rochala]

I looked at this, and I think it didn't solve the issue I've had, but the reason is completely on me. I made a mistake when I reported it. Likewise, I have updated the comment on the issue with the real reproduction. The fix should be similar, though.

[natsukagami]

Superceded by #21856