Add llvm 21 patch #2843

Job	Run time
osx26-arm-clang-repl-20-emscripten	12s
ubu24-arm-clang-repl-20-emscripten	15s
win2025-x86-clang-repl-20-emscripten	16s
ubu24-x86-clang-repl-20-emscripten	10s
osx26-arm-clang-repl-20-emscripten_wasm	1h 32m 30s
ubu24-arm-clang-repl-20-emscripten_wasm	49m 47s
ubu24-x86-clang-repl-20-emscripten_wasm	53m 0s
win2025-x86-clang-repl-20-emscripten	1h 39m 26s
	4h 55m 36s

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