LLVM submodule update

[frej]

Changes needed:

• There is no longer a way to define a custom verifer in tablegen using verifier. Instead a verifier has to use the default signature, and its existance is indicated by hasVerifier.

• Upstream has split out AttrDef/TypeDef and pattern constructs from OpBase.td, so we have to include mlir/IR/PatternBase.td when defining rewrite patterns using Tablegen.

• Adapt to upstream refactoring and moving function anc call interfaces to their own headers.

• The attribute used for the type of a function has been renamed to function_type from type for operators using the function interface. So we have to do the corresponding rename for the Rust dialect and update the lowering of Arc to Rust.

[frej]

Pinning to an upstream version is a good way to slowly slide into irrelevance. There are already too many interesting projects which say "unfortunately this requires an old LLVM version, but we are working on a forward port" and are never heard from again. By not staying up to date we would not benefit from upstream improvements. If we find bugs, upstream would not be interested in fixing them unless we could show that they are present in HEAD.

By syncing with upstream once a week we stay relevant and by doing it weekly there is a limited number of upstream commits to go through to find what has broken our code.