We do something similar, except we add x86_64-apple-darwin to our LLVM_BUILTIN_TARGETS and LLVM_RUNTIME_TARGETS explicitly. Clang has special logic for overriding darwin triples with the appropriate OS and architecture, and the Apple runtime builds do so. ⚙ D86313 [runtimes] Allow LLVM_BUILTIN_TARGETS to include Darwin has some related discussion.