Create Fortran bindings module

[joewallwork]

Closes #2805

Thanks to @isaacaka for figuring out how to make use of activity descriptors in Fortran: bind to those that Enzyme picks up in C. To avoid users having to know how to do Fortran-C binding, we thought it would be beneficial to create a Fortran module containing these for convenience. There will likely be other things that come up that we can add to the module later in the project.

To use these descriptors in Fortran, import them with

use enzyme_mod, only: enzyme_const, enzyme_dup

or just

use enzyme_mod

Checklist

• Add a Fortran module into the repo containing these bindings
• Hook it up in the build system

[vchuravy]

Is use enzyme_mod common in Fortran? Would it make more sense to just have use enzyme?

[joewallwork]

Is use enzyme_mod common in Fortran? Would it make more sense to just have use enzyme?

Both are common although I suppose _mod tends to be appended in older codes. I'm happy to use enzyme instead.

[joewallwork]

Is use enzyme_mod common in Fortran? Would it make more sense to just have use enzyme?

Both are common although I suppose _mod tends to be appended in older codes. I'm happy to use enzyme instead.

Done in 27b7edc.

[wsmoses]

what does this do?

[joewallwork]

Without adding Fortran to the languages in the project, I get the CMake error

FortranCInterface requires the Fortran language to be enabled

when building Enzyme. If I remove that from Fortran/CMakeLists.txt then I get

CMake can not determine linker language for target: EnzymeFortran

[joewallwork]

Fixed in 0479c37 by enabling Fortran in the subdir rather than globally.

[joewallwork]

Also added a CMake option ENZYME_FORTRAN in 827e439 that's defaulted to off like the other Fortran-related options. Turning it on builds the bindings.