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
• Need to verify performance, as the manual inliner has been disabled and the entire optimization pipeline is brand new (no issue yet)

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.