Expose location as part of LayoutConstraint for public access

Source/LayoutConstraint.swift

@@ -34,6 +34,9 @@ public class LayoutConstraint: NSLayoutConstraint {
     
     internal var snp_constraint: Constraint? = nil
     
+    public var snp_location: SourceLocation? {
+        return snp_constraint?.location
+    }
 }
 
 internal func ==(left: LayoutConstraint, right: LayoutConstraint) -> Bool {