swift234

Re-introduce the notion of implicit lvalues, to be used within the
constraint-based type checker. We now model the address-of operator
with the constraint

  [byref] T1 < T2

where T1 is a fresh variable and T2 is the type of the subexpression
of &. 

Note that there's a little hack in the constraint generator that
strips off the 'heap' qualifier when performing constraint-based type
checking, because we're not actually using it for overloading (merely
as an aid for IRgen) and it causes some trouble in the modeling of the
address-of operator.

We can now properly reject code such as

  swap(a, b)

which should be

  swap(&a, &b)



Swift SVN r2797