[llvm-c] Add EmulatedTLS and EnableTLSDESC to LLVMTargetMachineOptions

[ysbaddaden]

Adds a couple functions to LLVM-C to expose the Thread Local Storage options that are only available though the C++ API for now:

• LLVMTargetMachineOptionsSetEmulatedTLS
• LLVMTargetMachineOptionsSetEnableTLSDESC

We plan to use these options in @crystal-lang to reliably compile and cross compile to some targets regardless of downstream patches in OpenBSD or MingGW for example.

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[nikic]

Can you please add a release note entry under https://github.com/llvm/llvm-project/blob/main/llvm/docs/ReleaseNotes.md#changes-to-the-c-api?

[ysbaddaden]

@nikic Of course. I just did. I'm not so sure about the phrasing, though.

[nikic]

Hm... I think these need to be std::optional and only used if the corresponding functions are called? Otherwise we're not going to respect hasDefaultEmulatedTLS and hasDefaultTLSDESC anymore, right?

[ysbaddaden]

I investigated a bit: LLVMCreateTargetMachineWithOptions uses the default llvm::TargetOptions constructor that explicitly initializes both options to false already. Setting them again to false again is a NOOP.

I understand that the frontend is responsible to call hasDefaultEmulatedTLS or hasDefaultTLSDESC as a fallback value when unspecified. Clang appears to do that.

Now, I could change both options to be std::optional and have LLVMCreateTargetMachineWithOptions set the options when they have been explicit set, and fallback to hasDefaultEmulatedTLS and hasDefaultTLSDESC otherwise.