Auto merge of #127513 - nikic:llvm-19, r=cuviper

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 rust-lang/rust#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:
 * rust-lang/rust#127605
 * rust-lang/rust#127613
 * rust-lang/rust#127654
 * rust-lang/rust#128141
 * llvm/llvm-project#98933

Fixes rust-lang/rust#121444.
Fixes rust-lang/rust#128212.

Author: bors