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To avoid missing symbols when mixing newer LLVM headers with older Apple libc++, inject a __config_site that sets vendor availability annotations to match system library.

A hacky proof-of-concept to increase usability of ENV.llvm_clang without LLVM runtime dependency when bottling older macOS.

Also can use this PR to discuss other ideas for improving ENV.llvm_clang to avoid having to force linkage even when unnecessary.

Though we may want to figure out a solution for LLVM formula that works outside of brew since we always get user issues when new LLVM introduces annotation (some that I recall include LLVM 15 verbose abort, LLVM 18 std::print and LLVM 21 std::__hash_memory)