[lldb/CMake] Make check-lldb-* work for the standalone build.

In order to run check-lldb-* we need the correct map_config directives
in llvm-lit. For the standalone build, LLVM doesn't know about LLDB, and
the lldb mappings are missing. In that case we build our own llvm-lit,
and tell LLVM to use the llvm-lit in the lldb build directory.

Differential revision: https://reviews.llvm.org/D76945