[cmake] Add option to skip building lldb-server

Summary:
There is currently a way to skip the debugserver build. See how the CMake
variables SKIP_DEBUGSERVER and LLDB_CODESIGN_IDENTITY are used if you're
interested in that.

This allows us to skip building lldb-server as well. There is another
debug server called ds2 that can be used with LLDB. If you choose to use
ds2, this flag is very useful because it can cut down the build time of LLDB.

Differential Revision: https://reviews.llvm.org/D49282