swift234

Start tracking the how individual constraints map back to source expressions in the constraint solver.

We use uniqued "locators" to describe how a particular constraint was
derived, starting from the constraints generated on expressions (e.g.,
the argument to a function application must be convertible to the
input type of the function being applied) and narrowing down to
specific parts of the types (e.g., the result type of tuple element
2).

Note that locators are not present in all constraints yet, nor are
they actually used for anything other than slightly-more-interesting
debug dumps.



Swift SVN r4353