Header guard canonicalization, clang part.

Modifications made by clang-tidy with minor tweaks.

llvm-svn: 215557
diff --git a/clang/tools/diagtool/DiagTool.h b/clang/tools/diagtool/DiagTool.h
index b1e69f3..04b926d 100644
--- a/clang/tools/diagtool/DiagTool.h
+++ b/clang/tools/diagtool/DiagTool.h
@@ -11,8 +11,8 @@
 //
 //===----------------------------------------------------------------------===//
 
-#ifndef DIAGTOOL_DIAGTOOL_H
-#define DIAGTOOL_DIAGTOOL_H
+#ifndef LLVM_CLANG_TOOLS_DIAGTOOL_DIAGTOOL_H
+#define LLVM_CLANG_TOOLS_DIAGTOOL_DIAGTOOL_H
 
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/ManagedStatic.h"