Tracking LLVM workarounds

[workingjubilee]

This is a metabug for tracking the compiler's workarounds of LLVM's behavior that have resulted in the compiler implementing what one might refer to as "ugly hacks" to fix the behavior in question on our side. Logically, when the behavior is resolved and appropriate tests if any have been added, the issue can be closed, from Rust's perspective. Worse, leaving "fixed but not ideally" issues open makes it harder to sift through and prioritize problems. However, also logically, we also want to obtain various code improvements in the future, and not tracking them in some way makes it harder for us to find them again. Thus the purpose of this metabug is to allow observing these as a collective.

• Simple cases of std::iter::Iterator::any should yield asm equivalent to for loops, but are not #43517 patched by rustc: don't use union layouts for tagged union enums. #47007
• RUSTFLAGS="-Z cf-protection=full" gives prologue in naked functions for x86_64-unknown-none #98768 patched by Remove branch target prologues from #[naked] fn #98998