cmConstStack: Factor out of cmListFileBacktrace

This presents value semantics for a stack of constant values.
Internally it shares ownership to avoid copies.  Previously
this was implemented by `cmListFileBacktrace` explicitly,
but the approach can be re-used for other kinds of stacks.

Source/cmInstallFileSetGenerator.h

@@ -8,11 +8,11 @@
 #include <vector>
 
 #include "cmInstallGenerator.h"
-#include "cmListFileCache.h"
 #include "cmScriptGenerator.h"
 
 class cmGeneratorTarget;
 class cmFileSet;
+class cmListFileBacktrace;
 class cmLocalGenerator;
 
 class cmInstallFileSetGenerator : public cmInstallGenerator