[frontend] Make InputKind::Filename the default InputKind for a SelectedInput.

Nearly all uses of this will be to specify a filename, so make that the default.

Swift SVN r12149

include/swift/Frontend/FrontendOptions.h

@@ -42,7 +42,8 @@ public:
   /// The kind of input which this SelectedInput represents.
   InputKind Kind;
 
-  SelectedInput(unsigned Index, InputKind Kind) : Index(Index), Kind(Kind) {}
+  SelectedInput(unsigned Index, InputKind Kind = InputKind::Filename)
+      : Index(Index), Kind(Kind) {}
 };
 
 /// Options for controlling the behavior of the frontend.