Add accessors for MCSubtargetInfo CPU and Feature tables

[Keruspe]

Summary

This is needed for -C target-cpu=help and -C target-feature=help in rustc

This patch has been around for quite a while in rust's llvm fork:

Author: Cameron Hart <[e-mail]>
Date: Sun Jul 10 23:55:53 2016 +1000

[Keruspe]

Fixed repository in diff

[bcain]

LGTM but you should probably get approval from a reviewer w/experience in MCSubtargetInfo.h.

llvm/include/llvm/MC/MCSubtargetInfo.h

   /// prefetching.
   ///
   virtual unsigned getMinPrefetchStride() const;
+
+  ArrayRef<SubtargetSubTypeKV> getCPUTable() const {
+    return ProcDesc;
+  }
+
+  ArrayRef<SubtargetFeatureKV> getFeatureTable() const {
+    return ProcFeatures;
+  }
 };
 
 } // end namespace llvm