[ModuleInliner] Factor out common code in InlineOrder.cpp (NFC)

This patch factors out common code in InlineOrder.cpp.

Without this patch, the model is to ask classes like SizePriority and
CostPriority to compare a pair of call sites:

  bool hasLowerPriority(const CallBase *L, const CallBase *R) const override {

while these priority classes have their own caches of priorities:

  DenseMap<const CallBase *, PriorityT> Priorities;

This model results in a lot of duplicate code like hasLowerPriority
and updateAndCheckDecreased.

This patch changes the model so that priority classes just have two
methods to compute a priority for a given call site and to compare two
previously computed priorities (as opposed to call sites).

Facilities like hasLowerPriority and updateAndCheckDecreased move to
PriorityInlineOrder along with the map from call sites to their
priorities.  PriorityInlineOrder becomes a template class so that it
can accommodate different priority classes.

Differential Revision: https://reviews.llvm.org/D134149