Auto merge of #135763 - nikic:llvm-20, r=cuviper

Update to LLVM 20

LLVM 20 GA is scheduled for March 11th. Rust 1.87 will be stable on May 15th.

* [x] rust-lang/rust#135764
* [x] rust-lang/rust#136134
* [x] rust-lang/compiler-builtins#752
* [x] llvm/llvm-project#125287
* [x] rust-lang/rust#136537
* [x] rust-lang/rust#136895
* [x] Wait for beta branch (Feb 14).

Tested: host-x86_64, host-aarch64, apple, mingw, msvc