enhance silgen to treat OptionalSomePattern and EnumElementPattern as

"similar", avoiding false positive "not exhaustive" diagnostics on switches
like:

switch ... {
case let x?: break
case .None: break
}

Also, start using x? patterns in the stdlib more (review appreciated!), which 
is what shook this issue out.



Swift SVN r26004

stdlib/public/core/Algorithm.swift

@@ -151,7 +151,7 @@ public func split<S: Sliceable, R:BooleanType>(
   }
 
   switch startIndex {
-  case .Some(var i):
+  case let i?:
     result.append(elements[i..<elements.endIndex])
   default:
     ()
@@ -408,10 +408,7 @@ public func lexicographicalCompare<
       }
       return false
     }
-    switch e2_ {
-    case .Some(_): return true
-    case .None: return false
-    }
+    return e2_ != nil
   }
 }