Add accessors for MCSubtargetInfo CPU and Feature tables

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

Author: bitshifter

llvm/include/llvm/MC/MCSubtargetInfo.h

@@ -172,10 +172,19 @@ class MCSubtargetInfo {
     return Found != ProcDesc.end() && StringRef(Found->Key) == CPU;
   }
 
+
   /// Returns string representation of scheduler comment
   virtual std::string getSchedInfoStr(MCInst const &MCI) const {
     return {};
   }
+
+  ArrayRef<SubtargetFeatureKV> getCPUTable() const {
+    return ProcDesc;
+  }
+
+  ArrayRef<SubtargetFeatureKV> getFeatureTable() const {
+    return ProcFeatures;
+  }
 };
 
 } // end namespace llvm