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/StringUnicodeScalarView.swift

@@ -170,7 +170,7 @@ extension String {
         if _baseSet {
           if _ascii {
             switch self._asciiBase.next() {
-            case let .Some(x):
+            case let x?:
               result = .Result(UnicodeScalar(x))
             case .None:
               result = .EmptyInput