Support LLVM 15

[HertzDevil]

🎊

Part of #12743. This PR removes the remaining explicit uses of typed pointers, and also adds a few other things that are not exactly related to opaque pointers.

The manual specs under spec/llvm-ir are not adjusted for LLVM 15's opaque pointers yet. This is not that important because those specs' focus was never about pointers.

Additional cleanup and refactoring is still needed before this could be merged into the main branch.

[HertzDevil]

On my M2 and my ARM64 Alpine Linux VM, retrieving the call stack will sometimes fail:

def foo(x)
  y = x
  if y
    if y < 0
      p caller
    end
  else
    return 12
  end
end

foo(-1)

Invalid memory access (signal 10) at address 0x1008c58a0
[0x100848e84] *Exception::CallStack::print_backtrace:Nil +104 in /Users/quinton/.cache/crystal/crystal-run-small.tmp
[0x100834b10] ~procProc(Int32, Pointer(LibC::SiginfoT), Pointer(Void), Nil)@src/signal.cr:127 +320 in /Users/quinton/.cache/crystal/crystal-run-small.tmp
[0x18b9802a4] _sigtramp +56 in /usr/lib/system/libsystem_platform.dylib
[0x196fd3878] _ZN9libunwind12UnwindCursorINS_17LocalAddressSpaceENS_15Registers_arm64EE4stepEv +1152 in /usr/lib/system/libunwind.dylib
[0x196fd60f0] _Unwind_Backtrace +348 in /usr/lib/system/libunwind.dylib
[0x100835f20] *Exception::CallStack::unwind:Array(Pointer(Void)) +80 in /Users/quinton/.cache/crystal/crystal-run-small.tmp
[0x100835ebc] *Exception::CallStack#initialize:Array(Pointer(Void)) +20 in /Users/quinton/.cache/crystal/crystal-run-small.tmp
[0x100835e94] *Exception::CallStack::new:Exception::CallStack +48 in /Users/quinton/.cache/crystal/crystal-run-small.tmp
[0x100831964] *caller:Array(String) +16 in /Users/quinton/.cache/crystal/crystal-run-small.tmp

I have no idea what is going on. This was built using Homebrew's LLVM package because LLVM 15 is currently unavailable in any stable nixpkgs archive. The same crash isn't reproducible on x86-64 Linux.

EDIT: This is apparently fixed by changing the uwtable attribute argument to Sync.