[llvm][ModuleInliner] Refactor InlineSizePriority and PriorityInlineOrder

This patch introduces the abstract base class InlinePriority to serve as
the comparison function for the priority queue.  A derived class, such
as SizePriority, may choose to cache the priorities for different
functions for performance reasons.

This design shields the type used for the priority away from classes
outside InlinePriority and classes derived from it.  In turn,
PriorityInlineOrder no longer needs to be a template class.

Reviewed By: kazu

Differential Revision: https://reviews.llvm.org/D126300