[ADT] Fix MapVector when 'Map::mapped_type != unsigned'.

Previously MapVector assumed `Map::mapped_type` was `unsigned`.
This caused problems when using MapVector with a user-specified
map where this didn't hold (For example StringMap<unsigned>).

This patch adjusts MapVector to use the same type as the underlying
map, avoiding reference binding errors in functions like `insert`.

llvm-svn: 329523