Split ScarinessScore between its "storage" (POD), and an initializing object.

Summary:
This is needed so we can use it for D23672 on VS2013, since this VS
version doesn't support unrestricted unions, and doesn't allow us to
uses an object without a trivial default constructor inside a union.

Reviewers: kcc, samsonov

Subscribers: kubabrecka, llvm-commits

Differential Revision: https://reviews.llvm.org/D24029

llvm-svn: 280110