Compiler API: implement linking with installed-LLVM

[bjacob]

When built with installed-LLVM (a.k.a. bring-your-own-LLVM, as opposed to the default bundled-LLVM build), the Compiler API currently disables linking, as a runtime error:
https://github.com/openxla/iree/blob/c457c30c0ed86f42b76f18eb7fa160d6daeba49a/compiler/src/iree/compiler/API/Internal/LLDToolEntryPoint.cpp#L12-L21

This IREE_COMPILER_LLD_DISABLED token is defined in the build when IREE_LLD_TARGET is falseish,
https://github.com/openxla/iree/blob/c457c30c0ed86f42b76f18eb7fa160d6daeba49a/compiler/src/iree/compiler/API/Internal/CMakeLists.txt#L95-L111

Which happens when building with installed-LLVM, since IREE_LLD_TARGET is only defined in the bundled-LLVM case.

[ScottTodd]

Oh, I had some state on this with #12583 and related issues/discussions (#12830, #12884). I've context switched out much of that though...

[Evan-Zhao]

Curious what's the status on this now? I'm on commit #675aafb56 (18 hours ago) and ran into the same problem.

[stellaraccident]

I wasn't aware that we had left this dangling issue. I think some patches were landed which probably fixed the mechanism but probably neglected to get it over the line completely.

[ScottTodd]

#15580 made LLD optional, but I still see a few "byo llvm" tests trying to link with it and failing (on #15878, logs: https://github.com/openxla/iree/actions/runs/7171206985/job/19525680395?pr=15878#step:4:18074).

[Evan-Zhao]

#15580 made LLD optional, but I still see a few "byo llvm" tests trying to link with it and failing (on #15878, logs: https://github.com/openxla/iree/actions/runs/7171206985/job/19525680395?pr=15878#step:4:18074).

I see that merge request (good to have!), but IREE_COMPILER_LLD_DISABLED is still around as a separate TODO item on this line. Do we have that done somewhere waiting to be merged? If not, maybe I can go and try to figure this out?