adding codegen test to autodiff

[KMJ-007]

r? @ZuseZ4

[ZuseZ4]

You'll need the PrintTA flag, look in the rustc-dev-guide for the exact spelling. That should at least give you some typetree metadata.

You likely also want to first merge support for the more specific flag we discussed where you can pass a specific function name. Look into enzyme to find out the name of the flag and grep for how enzyme handles it internally. We likely want to pass that flag to enzyme not via the command line, since we call libEnzyme, which doesn't accept command line flags.
Instead, you should pass it just like all the other flags I'm passing to Enzyme, you can find the handling in rustc_codegen_llvm/src/back/write/lto.rs. First try to pass a hard-coded function name to enzyme, and make it print TA just for that function. We can later generalize it.

I'd focus for the next days on first getting the flags tested and supported, then you can continue with the tests here.