Give the MCStreamer class hierarchy LLVM RTTI facilities for use with

isa<> and dyn_cast<>. In several places, code is already hacking around
the absence of this, and there seem to be several interfaces that might
be lifted and/or devirtualized using this.

This change was based on a discussion with Jim Grosbach about how best
to handle testing for specific MCStreamer subclasses. He said that this
was the correct end state, and everything else was too hacky so
I decided to just make it so.

No functionality should be changed here, this is just threading the kind
through all the constructors and setting up the classof overloads.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174113 91177308-0d34-0410-b5e6-96231b3b80d8

lib/MC/MCNullStreamer.cpp

@@ -19,7 +19,7 @@ namespace {
 
   class MCNullStreamer : public MCStreamer {
   public:
-    MCNullStreamer(MCContext &Context) : MCStreamer(Context) {}
+    MCNullStreamer(MCContext &Context) : MCStreamer(SK_NullStreamer, Context) {}
 
     /// @name MCStreamer Interface
     /// @{
@@ -109,6 +109,11 @@ namespace {
     }
 
     /// @}
+
+    static bool classof(const MCStreamer *S) {
+      return S->getKind() == SK_NullStreamer;
+    }
+
   };
 
 }