IWYU: Update CMake code for IWYU built with Clang 6

IWYU now correctly requires `<utility>` for `std::move`.  It also
requires a container header when used via a range-based for loop.

Source/cmExportInstallFileGenerator.cxx

@@ -4,6 +4,7 @@
 
 #include "cmAlgorithms.h"
 #include "cmExportSet.h"
+#include "cmExportSetMap.h"
 #include "cmGeneratedFileStream.h"
 #include "cmGeneratorExpression.h"
 #include "cmGeneratorTarget.h"
@@ -21,8 +22,6 @@
 #include <sstream>
 #include <utility>
 
-class cmExportSetMap;
-
 cmExportInstallFileGenerator::cmExportInstallFileGenerator(
   cmInstallExportGenerator* iegen)
   : IEGen(iegen)