[adt] SparseBitVector::test() should be const

Summary:
Re-worked SparseBitVector's most-recently-used-word caching (CurrElementIter)
such that SparseBitVector::test() can be made const. This came up when
attempting to test individual bits in a SparseBitVector which was a member of a
const object.

The cached iterator has no bearing on the externally visible state, it's merely
a performance optimization. Therefore it has been made mutable and
FindLowerBound() has been split into a const and non-const function
(FindLowerBound/FindLowerBoundConst) for the const/non-const
interfaces.

Reviewers: rtereshin

Reviewed By: rtereshin

Subscribers: rtereshin, dexonsmith, kristina, llvm-commits

Differential Revision: https://reviews.llvm.org/D53447

llvm-svn: 345772