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

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Topic-rename: cmDebugTools-add-header