ADT: Add MapVector::remove_if

Add a `MapVector::remove_if()` that erases items in bulk in linear time,
as opposed to quadratic time for repeated calls to `MapVector::erase()`.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213090 91177308-0d34-0410-b5e6-96231b3b80d8