Revise include order using clang-format-6.0

Run the `clang-format.bash` script to update our C and C++ code to a new
include order `.clang-format`.  Use `clang-format` version 6.0.

Source/cmExportBuildFileGenerator.cxx

@@ -2,6 +2,12 @@
    file Copyright.txt or https://cmake.org/licensing for details.  */
 #include "cmExportBuildFileGenerator.h"
 
+#include <map>
+#include <memory>
+#include <set>
+#include <sstream>
+#include <utility>
+
 #include "cmExportSet.h"
 #include "cmGeneratorExpression.h"
 #include "cmGeneratorTarget.h"
@@ -16,12 +22,6 @@
 #include "cmTargetExport.h"
 #include "cmake.h"
 
-#include <map>
-#include <memory>
-#include <set>
-#include <sstream>
-#include <utility>
-
 class cmSourceFile;
 
 cmExportBuildFileGenerator::cmExportBuildFileGenerator()