SmallVector: Move, don't copy, elements to make space for an insertion.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210432 91177308-0d34-0410-b5e6-96231b3b80d8
SmallVector: More movable improvements - don't copy elements to make space when inserting repeated elements.

Also split and improve tests a bit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210433 91177308-0d34-0410-b5e6-96231b3b80d8