Auto merge of #121395 - nikic:update-llvm-21, r=cuviper

Update to LLVM 18.1.0 rc 4

Fixes https://github.com/rust-lang/rust/issues/120819.
Fixes https://github.com/rust-lang/rust/issues/121180.
Fixes https://github.com/rust-lang/rust/issues/121239.
Fixes https://github.com/rust-lang/rust/issues/121367.