[Sema] Simplify CheckConstraintSatisfaction. NFC

- Exit early when constraint caching is disabled.
- Use unique_ptr to manage temporary lifetime.
- Fix a typo in a comment (InsertPos instead of InsertNode).

The new code duplicates the forwarding call to CheckConstraintSatisfaction,
but reduces the number of interconnected if statements and simplifies lifetime
management.

This increases the overall readability.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D124923