cmDebugTools: add header

This provides a utility macro which prints out:

- location of the call;
- the expression being evaluated; and
- the value of the expression.

Evaluates to the value of the expression.

Inspired by Rust's `dbg!` macro.

See: https://doc.rust-lang.org/stable/std/macro.dbg.html

Source/cmDebugTools.h

@@ -0,0 +1,23 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+#pragma once
+
+#include <iostream>
+
+#define CM_DBG(expr) cm::dbg_impl(__FILE__, __LINE__, #expr, expr)
+
+namespace cm {
+
+namespace {
+
+template <typename T>
+T dbg_impl(const char* fname, int line, const char* expr, T value)
+{
+  std::cerr << fname << ':' << line << ": " << expr << " = " << value
+            << std::endl;
+  return value;
+}
+
+} // namespace
+
+} // namespace cm