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/cmGlobalXCodeGenerator.cxx

@@ -2,7 +2,6 @@
    file Copyright.txt or https://cmake.org/licensing for details.  */
 #include "cmGlobalXCodeGenerator.h"
 
-#include "cmsys/RegularExpression.hxx"
 #include <cassert>
 #include <cstdio>
 #include <cstring>
@@ -11,6 +10,8 @@
 
 #include <cm/memory>
 
+#include "cmsys/RegularExpression.hxx"
+
 #include "cmAlgorithms.h"
 #include "cmComputeLinkInformation.h"
 #include "cmCustomCommand.h"