[BOLT][NFC] Move getInliningInfo out of Inliner class

`getInliningInfo` is useful in other passes that need to check inlining
eligibility for some function. Move the declaration and InliningInfo definition
out of Inliner class. Prepare for subsequent use in ICP.

Reviewed By: rafauler

Differential Revision: https://reviews.llvm.org/D124899