Upgrade to REUSE 4.x and `REUSE.toml`

[pietroalbini]

REUSE 4.0.0 was recently released, which addresses all the issues we had with REUSE through the addition of REUSE.toml. We should upgrade the version of REUSE to it, and migrate from .reuse/dep5 to REUSE.toml in the repository.

Notable things of REUSE.toml that would help our use:

• It is possible to instruct REUSE to ignore all annotations it would discover in source files and only use the annotations present in REUSE.toml. While in most cases we would not want to do that (as annotations in source files themselves is probably more accurate), we do want to override LLVM's licensing information with the boilerplate defined in .reuse/dep5/REUSE.toml. Setting precedence = "override" in REUSE.toml for LLVM only would address this.

• REUSE now supports having multiple REUSE.tomls in a repository, which means that a submodule can now define its own licensing annotations. Before that, only the top-level .reuse/dep5 would be considered, and any other instance of that file in subdirectories or submodules would be ignored.

We should:

• Update the version of REUSE to 4.x.x
• Convert .reuse/dep5 into REUSE.toml
• Revert c6eb03b and mark the LLVM annotation with precedence = "override"

Tracking issue: #99414