MC/DC LLVM 19 intrinsics changes

[krasimirgg]

In LLVM 19 some details about MC/DC changed, removing and updating some intrinsics:

• https://github.com/llvm/llvm-project/commit/85a7bba7d28365ff98dae74f20ebf9f53d42023a In instrprof_mcdc_condbitmap_update intrinsic was dropped recently:
• https://github.com/llvm/llvm-project/pull/95496

I tried naively to remove the reference to the removed intrinsic https://github.com/rust-lang/rust/pull/126582, but that's incomplete. Will need more work to adapt to this it seems:

• https://github.com/llvm/llvm-project/pull/95496#issuecomment-2173006812

[krasimirgg]

@ZhuUx

[ZhuUx]

As far as I know, works on mcdc need to be done for adapting to llvm 19 are:

1. Eliminate the removed intrinsic function as you have done.
2. Change the way to update condition bits as the new mechanism.
3. Let ConditionId start from 0 rather than 1.

These changes are definite since they have been merged into llvm now, while still there are other changes may be introduced.

Given that 2 actually changes several arguments we passed to llvm and causes severe incompatibility, I'd wish to stop supporting mcdc on llvm-18 once if we switched to 19.

Edit. Other works have been done, but 2 introduces more break changes than I thought I'd need more time to adapt to it.

[Zalathar]

In the short-term, we can just adjust the tests to not run on LLVM 19.

After the in-tree LLVM has been updated to LLVM 19, then we can remove support for MC/DC under LLVM 18, and add support for MC/DC under LLVM 19.

(Unless someone wants to do the extra work of supporting MC/DC with LLVM 19 before the official upgrade.)

[veera-sivarajan]

@rustbot label -needs-triage +A-LLVM +T-compiler +C-enhancement

[workingjubilee]

After the in-tree LLVM has been updated to LLVM 19, then we can remove support for MC/DC under LLVM 18, and add support for MC/DC under LLVM 19.

This sounds acceptable if we defer stabilizing MC/DC until our minimum LLVM is 19+

[ZhuUx]

Filed #126733 to fix it and prepare for llvm 19.