SmallVector::erase: Assert that iterators are actually inside the vector.

SmallVector::erase: Assert that iterators are actually inside the vector.

The rationale here is that it's hard to write loops containing vector erases and
it only shows up if the vector contains non-trivial objects leading to crashes
when forming them out of garbage memory.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@160854 91177308-0d34-0410-b5e6-96231b3b80d8