Update to LLVM 19
The LLVM 19.1.0 final release is planned for Sep 3rd. The rustc 1.82 stable release will be on Oct 17th.
The unstable MC/DC coverage support is temporarily broken by this update. It will be restored by https://github.com/rust-lang/rust/pull/126733. The implementation changed substantially in LLVM 19, and there are no plans to support both the LLVM 18 and LLVM 19 implementation at the same time.
Compatibility note for wasm:
> WebAssembly target support for the `multivalue` target feature has changed when upgrading to LLVM 19. Support for generating functions with multiple returns no longer works and `-Ctarget-feature=+multivalue` has a different meaning than it did in LLVM 18 and prior. The WebAssembly target features `multivalue` and `reference-types` are now both enabled by default, but generated code is not affected by default. These features being enabled are encoded in the `target_features` custom section and may affect downstream tooling such as `wasm-opt` consuming the module, but the actual generated WebAssembly will continue to not use either `multivalue` or `reference-types` by default. There is no longer any supported means to generate a module that has a function with multiple returns.
Related changes:
* https://github.com/rust-lang/rust/pull/127605
* https://github.com/rust-lang/rust/pull/127613
* https://github.com/rust-lang/rust/pull/127654
* https://github.com/rust-lang/rust/pull/128141
* https://github.com/llvm/llvm-project/pull/98933
Fixes https://github.com/rust-lang/rust/issues/121444.
Fixes https://github.com/rust-lang/rust/issues/128212.
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