[CSClosure] Anchor for-in sequence constraints on sequence expression

This was an incorrect locator since constraints do belong to a sequence
expression and not to a contextual pattern.

(cherry picked from commit f210442)

Author: xedin

lib/Sema/CSClosure.cpp

@@ -440,7 +440,7 @@ class ClosureConstraintGenerator
 
     cs.addConstraint(
         ConstraintKind::Conversion, elementType, initType,
-        cs.getConstraintLocator(contextualLocator,
+        cs.getConstraintLocator(sequenceLocator,
                                 ConstraintLocator::SequenceElementType));
 
     // Reference the makeIterator witness.
@@ -449,9 +449,10 @@ class ClosureConstraintGenerator
 
     Type makeIteratorType =
         cs.createTypeVariable(locator, TVO_CanBindToNoEscape);
-    cs.addValueWitnessConstraint(LValueType::get(sequenceType), makeIterator,
-                                 makeIteratorType, closure,
-                                 FunctionRefKind::Compound, contextualLocator);
+    cs.addValueWitnessConstraint(
+        LValueType::get(sequenceType), makeIterator, makeIteratorType,
+        closure, FunctionRefKind::Compound,
+        cs.getConstraintLocator(sequenceLocator, ConstraintLocator::Witness));
 
     // After successful constraint generation, let's record
     // solution application target with all relevant information.