Driver: Eliminate Arg subclasses, which are now unnecessary.

llvm-svn: 105762
diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
index 90333bd..bd70982 100644
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -976,7 +976,7 @@
     // just using Args was better?
     const Arg &Input = IA->getInputArg();
     Input.claim();
-    if (isa<PositionalArg>(Input)) {
+    if (Input.getOption().matches(options::OPT_INPUT)) {
       const char *Name = Input.getValue(C.getArgs());
       Result = InputInfo(Name, A->getType(), Name);
     } else