librustc: Allow rvalues to be borrowed as &mut.

[luqmana]

#5967

[nikomatsakis]

This doesn't quite look right. I believe the correct thing to do is simply to modify the mem_categorization code to categorize rvalues as mutable. Only thing is that I am not sure if there are other places that assume that they are immutable, or if there will be other follow on changes needed. (Note: I had planned to do this as part of the code for #5074)

[catamorphism]

Needs a rebase.

[luqmana]

Rebased this but not sure if @nikomatsakis has anything as part of the new borrowck.

[luqmana]

Closed in favour of #6473.