add IWYU pragmas for recursive includes (e.g. implementation headers)

[BenWibking]

Summary

It would be helpful to add Include What You Use (IWYU) pragmas to indicate whether certain header files should not be included by the user directly. This is most often the case for *_impl.hpp header files that provide implementations for template classes.

This is a problem because many IDEs will auto-include headers incorrectly without these pragmas (also, clangd and other tools will complain about violating include-what-you-use).

The two relevant ones are export and private:

foo.h:

// end of file
#include "foo_impl.h" // IWYU pragma: export

foo_impl.h:

// beginning of file
// IWYU pragma: private; include "foo.h"

Docs: https://github.com/include-what-you-use/include-what-you-use/blob/master/docs/IWYUPragmas.md

Headers affected

These are the headers that could have such pragmas included:

• https://github.com/AMReX-Codes/amrex/blob/development/Src/Base/AMReX_GpuLaunch.H
• any others?