Change SILParser::parseSILIdentifierSwitch to use the template parameter.

In `parseSILIdentifierSwitch`, `Result` has the type of one of the template parameters. Without this patch we are requiring `T` to be a `ValueOwnershipKind` (or convertible to that), which makes the function not usable on other result types and defeats the point of having a templated argument.

lib/ParseSIL/ParseSIL.cpp

@@ -264,7 +264,7 @@ namespace {
         return true;
       }
 
-      Result = ValueOwnershipKind(*Iter);
+      Result = T(*Iter);
       return false;
     }