[BOLT] Fix buildbot failure

[maksfb]

Summary

GCC requires "class" keyword when variable name matches class name.

bolt/include/bolt/Rewrite/RewriteInstance.h

public:
 
   /// Adds Debug section to overwrite.
   static void addToDebugSectionsToOverwrite(const char *Section) {
     DebugSectionsToOverwrite.emplace_back(Section);
   }
 
 private:
   /// Manage a pipeline of metadata handlers.
-  MetadataManager MetadataManager;
+  class MetadataManager MetadataManager;
 
   /// Get the contents of the LSDA section for this binary.
   ArrayRef<uint8_t> getLSDAData();
 
   /// Get the mapped address of the LSDA section for this binary.
   uint64_t getLSDAAddress();
 
   static const char TimerGroupName[];