LLL is a low-level language for the EVM with an s-expressions syntax.
The Solidity repository contains an LLL compiler, which shares the assembler subsystem with Solidity. However, apart from maintaining that it still compiles, no other improvements are made to it.
It is not built unless specifically requested:
$ cmake -DLLL=ON .. $ cmake --build .
The LLL codebase is deprecated and will be removed from the Solidity repository in the future.