fix <rdar://problem/23798944> = vs. == in Swift if string character count statement causes segmentation fault

This is a really nasty problem where we'd explode trying to unify type variables
we found in the constraint graph, that didn't exist in the constraint system.  This
happened because diagnostic generation is simplifying the system in
"ContinueAfterFailures" mode to try to get to a minimal system (to avoid
producing an error message about trivially solvable constraints that only
exist due to the solver stopping early) and in this mode we would see an
erroneous constraint, remove it from the constraint system, but not the
constraint graph.  This only mattered in ContinueAfterFailures.

As with many things, the fix is pretty simple once the onion is peeled enough
to find the stinky part lurking inside.

Author: lattner

lib/Sema/CSSolver.cpp

@@ -336,6 +336,8 @@ bool ConstraintSystem::simplify(bool ContinueAfterFailures) {
 
       if (solverState)
         solverState->retiredConstraints.push_front(constraint);
+      else
+        CG.removeConstraint(constraint);
 
       break;
 

test/Constraints/assignment.swift

@@ -49,3 +49,11 @@ func value2(inout x: Int) {}
 value2(&_) // expected-error{{'_' can only appear in a pattern or on the left side of an assignment}}
 value(_) // expected-error{{'_' can only appear in a pattern or on the left side of an assignment}}
 
+
+// <rdar://problem/23798944> = vs. == in Swift if string character count statement causes segmentation fault
+func f23798944() {
+  let s = ""
+  if s.characters.count = 0 { // expected-error {{cannot assign to property: 'count' is a get-only property}}
+  }
+}
+