rustc::hir::map::definitions - fix O(n^2) when disambiguating

rustc::hir::map::definitions - fix O(n^2) when disambiguating

Instead of finding the next free disambiguator by incrementing it until
you find a place, store the next available disambiguator in an hash-map.

This avoids O(n^2) performance when lots of items have the same
un-disambiguated `DefPathData` - e.g. all `use` items have
`DefPathData::Misc`.