`include-what-you-use` diagnostics, in practice, are specific to
the environment's compiler and standard library. Update includes
to satisfy IWYU for our CI job under Debian 13. Some patterns:
* Types named in virtual `override` signatures no longer require
includes since the overridden signature already names them.
* A function argument's type needs to be included even if its constructor
is called only by implicit conversion. For example, constructing a
`std::function` from a lambda now requires `<functional>`.
* Some prior mysterious `<type_traits>` inclusions are no longer required.
When CMAKE_FASTBUILD_USE_RELATIVE_PATHS is set -
use relative paths for Unity nodes as well as for include directories.
It makes it possible to share cache across different location of
the same repo (or potentially across different machines)
Eduard Voronkin
Brad King
Eduard Voronkin
Ben Boeckel