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.cxx

@@ -11,6 +11,7 @@
 #include "cmGeneratorExpression.h"
 #include "cmGlobalGenerator.h"
 #include "cmInstallType.h"
+#include "cmListFileCache.h"
 #include "cmLocalGenerator.h"
 #include "cmStringAlgorithms.h"