WIP: Upgrade to LLVM 21

[ausbin]

Posting as a draft PR because there are are some interesting build errors when I run cargo test in my static-build fork (I assume they'd happen here too):

error: macros that expand to items must be delimited with braces or followed by a semicolon
   --> melior/src/dialect/ods.rs:180:1
    |
180 | / melior_macro::dialect! {
181 | |     name: "tosa",
182 | |     files: ["mlir/Dialect/Tosa/IR/TosaOps.td"],
183 | | }
    | |_^
    |
    = note: this error originates in the macro `melior_macro::dialect` (in Nightly builds, run with -Z macro-backtrace for more info)

error: invalid ODS input: Included from :1:
       Included from /home/austin/bin/llvm21/include/mlir/Dialect/Tosa/IR/TosaOps.td:17:
       Included from /home/austin/bin/llvm21/include/mlir/IR/OpBase.td:18:
       Included from /home/austin/bin/llvm21/include/mlir/IR/Interfaces.td:16:
       Included from /home/austin/bin/llvm21/include/mlir/IR/AttrTypeBase.td:17:
       /home/austin/bin/llvm21/include/mlir/IR/CommonAttrConstraints.td:31:8: error: invalid conversion from Invalid to alloc::string::String
         code storageType = ?; // The backing mlir::Attribute type
              ^
   --> melior/src/dialect/ods.rs:180:1
    |
180 | / melior_macro::dialect! {
181 | |     name: "tosa",
182 | |     files: ["mlir/Dialect/Tosa/IR/TosaOps.td"],
183 | | }
    | |_^
    |
    = note: this error originates in the macro `melior_macro::dialect`

However, I hope this PR still saves the maintainers a few minutes of time, particularly the updated pass names.

Requires mlir-rs/mlir-sys#71 and mlir-rs/tblgen-rs#28 (or equivalent changes)