[Clang] Missing attribute to diagnose unused variables with certain constructors

There are some classes (e.g. `std::unique_lock`), which only do something meaningful when certain constructors are used. Others don't do anything and should be diagnosed. Unfortunately, there is currently no equivalent to `[[gnu::warn_unused]]` for individual constructors.

Start by locating the existing implementation of the [[gnu::warn_unused]] attribute and the diagnostics for std::unique_lock. Determine how an equivalent attribute could apply to individual constructors, including which constructors should trigger a warning and which should not. Done means the attribute is supported and the intended constructor-specific diagnostics are covered.

Written by the indexing model from the issue text.