Tracking issue for the LLVM 20 upgrade

[alexrp]

We've accumulated a number of issues that should be addressed with LLVM 20, so just opening this as a meta issue to track them all in one place:

• [ ] https://github.com/ziglang/zig/issues/21051
• [ ] https://github.com/ziglang/zig/issues/21096
• [ ] https://github.com/ziglang/zig/issues/21215
• [ ] https://github.com/ziglang/zig/issues/21329
• [ ] https://github.com/ziglang/zig/issues/21411
• [ ] https://github.com/ziglang/zig/issues/21630
• [ ] https://github.com/ziglang/zig/issues/21818
• [ ] https://github.com/ziglang/zig/issues/22003
• [ ] https://github.com/ziglang/zig/blob/a9c7714b784eeb5e57f6bc718c1e621461dca532/src/codegen/llvm.zig#L273-L274
• [ ] https://github.com/ziglang/zig/blob/a9c7714b784eeb5e57f6bc718c1e621461dca532/src/target.zig#L466-L474
• [ ] https://github.com/ziglang/zig/blob/a9c7714b784eeb5e57f6bc718c1e621461dca532/lib/std/Target.zig#L2266-L2273