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The LLVM IR doesn't define an ABI - in particular, most of the ABI is already "burned in" at the point where LLVM IR exists.

Not obviously. For example, all of those languages mangle names differently.

LLVM has so many little ways to configure its output, and unless two LLVM users configure the outputs to be compatible, they are unlikely to be. LLVM is more of a toolset with some language than a shared infrastructure.