It will NOT reimplement everything, if you read the proposal, it's in favour of changing the LLVM dependency (the libs) not dropping LLVM IR generation, this will come with performance regression since now will be up to the team to get the correct IR, and making decisions LLVM IR generation does already in LLVM
The problem is that clang is being dropped, which means, unless we have a new C++ front made in zig (a-la AroCC for C) we are gonna suffer quite a bit for projects using C++ with zig.