[MC] Make MCELFStreamer::finishImpl final

This function is final after efdb91e.
Target-specific code should override MCTargetStreamer::finish instead,
e.g. AArch64TargetELFStreamer::finish
(fec1b6f).

Author: MaskRay

llvm/include/llvm/MC/MCELFStreamer.h

@@ -80,7 +80,9 @@ class MCELFStreamer : public MCObjectStreamer {
   void emitCGProfileEntry(const MCSymbolRefExpr *From,
                           const MCSymbolRefExpr *To, uint64_t Count) override;
 
-  void finishImpl() override;
+  // This is final. Override MCTargetStreamer::finish instead for
+  // target-specific code.
+  void finishImpl() final;
 
   void emitBundleAlignMode(Align Alignment) override;
   void emitBundleLock(bool AlignToEnd) override;