`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.
`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 12.
The new cmFileTime class stores the file modification time as an OS independent
nanosecond count. Its main use is to load and compare file modification times
in nanosecond and second resolution.
