The brew LLVM situation is crazy and even more noticeable when you're on older macOS for which they no longer ship precompiled stuff.

I know they don't want stuff breaking because users have a GCC version that's 6 months behind and the homebrew folks can't be sure that package XYZ doesn't use any newer features. But when installing anything built with C++ I really don't expect to wait half a day for the whole LLVM+CMake toolchain to compile from scratch.

You can pin LLVM but it's useless as there's no bypass to the version check. They've also closed all relevant issues as "won't fix" as it's not "their way of doing things".