Make MapVector a bit more expensive but harder to misuse. We now only

Make MapVector a bit more expensive but harder to misuse. We now only
provide insertion order iteration, instead of the old option of
DenseMap order iteration over keys and insertion order iteration over
values.
This is implemented by keeping two copies of each key.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@164221 91177308-0d34-0410-b5e6-96231b3b80d8