swift234

Add infrastructure for applying fixes during constraint solving.

Introduce some infrastructure that allows us to speculatively apply
localized fixes to expressions during constraint solving to fix minor
typos and omissions. At present, we're able to introduce the fixes
during constraint simplification, prefer systems with fewer fixes when
there are multiple fixes, and diagnose the fixes with Fix-Its.

Actually rewriting the AST to reflect what the Fix-Its are doing is
still not handled.

As a start, introduce a fix that adds '()' if it appears to have been
forgotton, producing a diagnostic like this if it works out:

t.swift:8:3: error: function produces expected type 'B'; did you mean
to call it with '()'?
f(g)
  ^
   ()

Note that we did regress in one test case
(test/NameBinding/multi-file.swift), because that diagnostic was
getting lucky with the previous formulation.

Swift SVN r16937