Constants.h: Fix possible typo in r216015. [-Wdocumentation]

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216043 91177308-0d34-0410-b5e6-96231b3b80d8
Fix an off by 1 bug that prevented SmallPtrSet from using all of its 'small' capacity. Then fix the early return in the move constructor that prevented 'small' moves from clearing the NumElements in the moved from object. The directed test missed this because it was always testing large moves due to the off by 1 bug.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216044 91177308-0d34-0410-b5e6-96231b3b80d8