Welcome to Ziggit!
Looks like the build process is trying to use Apple LLVM and homebrew LLVM at the same time? Using prebuilt LLVM versions is a bit of a pain especially on macOS since none of them seem to be fully suitable for building Zig.
I wouldn’t recommend using either to build Zig from source anyway, just use GitHub - ziglang/zig-bootstrap: take off every zig and compile everything from source. Works every time.