ADT: Fix MapVector::erase()

Actually update the changed indexes in the map portion of `MapVector`
when erasing from the middle.  Add a unit test that checks for this.

Note that `MapVector::erase()` is a linear time operation (it was and
still is).  I'll commit a new method in a moment called
`MapVector::remove_if()` that deletes multiple entries in linear time,
which should be slightly less painful.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213084 91177308-0d34-0410-b5e6-96231b3b80d8