ADT: Shrink size of SmallVector by 8B on 64-bit platforms

Representing size and capacity directly as unsigned and calculate
`end()` using `begin() + size()`.

This limits the maximum size/capacity of a vector to UINT32_MAX.

https://reviews.llvm.org/D48518

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@337504 91177308-0d34-0410-b5e6-96231b3b80d8

Author: dexonsmith