[Type checker] Look through reference storage types for typed patterns.

Fixes a regression caused by properly setting the context for checking
the type in a typed pattern.

Author: DougGregor

lib/Sema/CSGen.cpp

@@ -2328,6 +2328,10 @@ namespace {
             : ContextualPattern::forRawPattern(pattern, CurDC);
 
         Type type = TypeChecker::typeCheckPattern(contextualPattern);
+
+        // Look through reference storage types.
+        type = type->getReferenceStorageReferent();
+
         Type openedType = CS.openUnboundGenericType(type, locator);
 
         // Determine the subpattern type. It will be convertible to the