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Auto merge of #143684 - nikic:llvm-21, r=cuviper
Update to LLVM 21
Timeline: LLVM 21.1.0 is scheduled to release on Aug 26th. Rust 1.90 branches on Aug 1st and releases September 18.
Depends on:
* [x] https://github.com/llvm/llvm-project/issues/147781
* [x] https://github.com/llvm/llvm-project/issues/147935
* [x] https://github.com/llvm/llvm-project/issues/139443
* [x] https://github.com/llvm/llvm-project/pull/148207
* [x] https://github.com/llvm/llvm-project/pull/148607
* [x] https://github.com/llvm/llvm-project/pull/149046
* [x] https://github.com/llvm/llvm-project/issues/149097
* [x] https://github.com/rust-lang/rust/pull/144116
r? `@ghost`