Known issues in LLVM 17

[elliottslaughter]

Since we've got a couple of them, listing all the (new) known issues in LLVM 17:

• constantinits.t breaks on LLVM 17 Linux #644
• Compile time regression in compile_time_array.t on LLVM 17 #651
• Nondeterministic failure in fakeasm.t in LLVM 17 on Linux #654
• setinlined not working in JIT mode on LLVM >= 17 #671
• Need to verify performance, as the manual inliner has been disabled and the entire optimization pipeline is brand new (no issue yet)
• I have seen performance regressions in CUDA applications on LLVM 16 vs 18, need to investigate and file a formal issue

It's frankly not obvious if these are worth investigating. The transition to opaque pointers shook looks a bunch of problems in LLVM that were not investigated prior to the release. We may just find that some of these go away with LLVM 18 or subsequent versions.