Diagnostics: Ensure state is initialized

Tweak how we define the storage type for diagnostic state to ensure that
the memory is always initialized.

Source/cmDiagnostics.h

@@ -101,7 +101,11 @@ public:
     cm::string_view name);
 
   /** Represent a set of diagnostic category actions.  */
-  using DiagnosticMap = std::array<DiagnosticAction, CategoryCount>;
+  struct DiagnosticMap : public std::array<DiagnosticAction, CategoryCount>
+  {
+    DiagnosticMap()
+      : array{} {};
+  };
 };
 
 using cmDiagnosticCategory = cmDiagnostics::DiagnosticCategory;

Source/cmMakefile.cxx

@@ -5,6 +5,7 @@
 #include "cmMakefile.h"
 
 #include <algorithm>
+#include <array>
 #include <cassert>
 #include <cstdio>
 #include <cstdlib>

Source/cmMessenger.cxx

@@ -3,6 +3,7 @@
 #include "cmMessenger.h"
 
 #include <algorithm>
+#include <array>
 #include <sstream>
 #include <utility>