Devirtualize File::kind.

This is the standard way of implementing LLVM RTTI.

llvm-svn: 177555
diff --git a/lld/lib/ReaderWriter/ReaderLinkerScript.cpp b/lld/lib/ReaderWriter/ReaderLinkerScript.cpp
index 78203fa..4beb32a 100644
--- a/lld/lib/ReaderWriter/ReaderLinkerScript.cpp
+++ b/lld/lib/ReaderWriter/ReaderLinkerScript.cpp
@@ -30,8 +30,6 @@
     return std::move(file);
   }
 
-  virtual Kind kind() const { return kindLinkerScript; }
-
   static inline bool classof(const File *f) {
     return f->kind() == kindLinkerScript;
   }
@@ -64,7 +62,7 @@
 
 private:
   LinkerScriptFile(const TargetInfo &ti, std::unique_ptr<llvm::MemoryBuffer> mb)
-      : File(mb->getBufferIdentifier()),
+      : File(mb->getBufferIdentifier(), kindLinkerScript),
         _targetInfo(ti),
         _lexer(std::move(mb)),
         _parser(_lexer),