Create placeholder symbols for checking callee and improve TypeVar instantiation logic

[noti0na1]

This PR refines the TypeVar instantiation logic introduced by #24231.

We only check the calleeType when there is actually a TypeVar in pt to instantiate now, prioritizing inference from the callee.

Close #24686 #24689 #24694 #24696

Maybe #24695, since the behaviour is the same as 3.3 and 3.7 now.

calleeType itself still has some issues, will fix in a separate PR: #24732