Add implicit interfaces for function hooks in Fortran module

[joewallwork]

Closes #2808.
Merges into #2807.

As mentioned in the issue, it turns out that the interfacing to the special Enzyme function hooks can be done using implicit interfaces in Fortran. That is, you can get away without providing an explicit interface block like is done in the examples so long as you declare

external :: __enzyme_autodiff

for example.

Checklist

• Add to Fortran enzyme module
• Use in square example

Hooking up in the allocatableArraySimple example will require fixing #2820.

[joewallwork]

Huh, looks like ifx doesn't like externals with leading underscores. I hadn't tested this with that compiler yet (just flang).

[joewallwork]

Huh, looks like ifx doesn't like externals with leading underscores. I hadn't tested this with that compiler yet (just flang).

Fixed in 237ebc6 by prepending with f for the Fortran versions of the function hooks.