[BuilderTransform] Use correct locator for case subject expressions

Instead of `CTP_Initialization` transform should use `CTP_CaseStmt`
to identify the root correctly.

Author: xedin

lib/Sema/BuilderTransform.cpp

@@ -758,7 +758,7 @@ class BuilderClosureVisitor
     // If needed, generate constraints for everything in the case statement.
     if (cs) {
       auto locator = cs->getConstraintLocator(
-          subjectExpr, LocatorPathElt::ContextualType(CTP_Initialization));
+          subjectExpr, LocatorPathElt::ContextualType(CTP_CaseStmt));
       Type subjectType = cs->getType(subjectExpr);
 
       if (cs->generateConstraints(caseStmt, dc, subjectType, locator)) {