InlineRefactoring: introducing InlinePolicy

InlineRefactoring: introducing InlinePolicy

Split the policy aspect of the InlineResult into a separate
policy object, acessed from the InlineResult via a base class.

Implement a LegacyPolicy to capture today's behavior.

Create a factory method for determining what policy to use. Right
now it always creates a LegacyPolicy. This is newed up for each
Inline decision, but since the InlineResult has relatively short
lifetime, we should probably recycle policy objects (might be
more desirable once they start having a bit more state).

Split out a bunch of the InlineDecision helper methods that lived
on InlineResult into global methods since policies will want to refer
to them too. Shame that enum classes don't allow member functions.