cmstd: Modernize CMake system headers

Provide a standardized way to handle the C++ "standard" headers
customized to be used with current CMake C++ standard constraints.
Offer under directory `cm` headers which can be used as direct
replacements of the standard ones.  For example:

    #include <cm/string_view>

can be used safely for CMake development in place of the `<string_view>`
standard header.

Fixes: #19491

Source/cmGeneratorExpressionEvaluationFile.h

@@ -10,9 +10,10 @@
 #include <string>
 #include <vector>
 
+#include "cm_sys_stat.h"
+
 #include "cmGeneratorExpression.h"
 #include "cmPolicies.h"
-#include "cm_sys_stat.h"
 
 class cmLocalGenerator;