[cmake] Use `CMAKE_INSTALL_LIBDIR` too

We held off on this before as `LLVM_LIBDIR_SUFFIX` conflicted with it.
Now we return this.

`LLVM_LIBDIR_SUFFIX` is kept as a deprecated way to set
`CMAKE_INSTALL_LIBDIR`. The other `*_LIBDIR_SUFFIX` are just removed
entirely.

I imagine this is too potentially-breaking to make LLVM 15. That's fine.
I have a more minimal version of this in the disto (NixOS) patches for
LLVM 15 (like previous versions). This more expansive version I will
test harder after the release is cut.

Reviewed By: sebastian-ne, ldionne, #libc, #libc_abi

Differential Revision: https://reviews.llvm.org/D130586