Has there been any refinement on a mission statement for it?

If I recall correctly from the mailing list when this was first proposed, someone had a use case for custom libc but extending it to a general purpose libc diminished properties that would distinguish it from (e.g.) musl-libc.

Wouldn't they just use Google's tcmalloc?

Probably not the ideal pick for an everuone facing library like this, tcmalloc focuses on reducing core thread contention, at the cost of compactness and often debugability. I've not been following the ptoject for some time though, so maybe things have changed. Still a great allocator if those tradoffs are what you want. Something more like mimalloc would probably be a better general purpose allocator for something like this

Users wanting allocator functions (malloc and friends) from LLVM libc will get the SCUDO allocator. They need to use a special CMake option to include SCUDO when building LLVM libc.

The plan is to use Scudo, which is the allocator that lives in compiler-rt.

Edited to include the link to Scudo: https://github.com/llvm/llvm-project/tree/main/compiler-rt/l...

Edited again: I am reminded that you can already get scudo today by using a CMake option to include scudo when building LLVM-libc.