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/cmConstStack.h

@@ -0,0 +1,39 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+#pragma once
+
+#include "cmConfigure.h" // IWYU pragma: keep
+
+#include <memory>
+
+/** Base class template for CRTP to represent a stack of constant values.
+    Provide value semantics, but use efficient reference-counting underneath
+    to avoid copies.  */
+template <typename T, typename Stack>
+class cmConstStack
+{
+  struct Entry;
+  std::shared_ptr<Entry const> TopEntry;
+
+public:
+  /** Default-construct an empty stack.  */
+  cmConstStack();
+
+  /** Get a stack with the given call context added to the top.  */
+  Stack Push(T value) const;
+
+  /** Get a stack with the top level removed.
+      May not be called until after a matching Push.  */
+  Stack Pop() const;
+
+  /** Get the value at the top of the stack.
+      This may be called only if Empty() would return false.  */
+  T const& Top() const;
+
+  /** Return true if this stack is empty.  */
+  bool Empty() const;
+
+protected:
+  cmConstStack(std::shared_ptr<Entry const> parent, T value);
+  cmConstStack(std::shared_ptr<Entry const> top);
+};