[CMake] Align debugserver with lldb-server on Darwin

Summary: Make debugserver a tool like lldb-server, so it can be included/excluded via `LLDB_TOOL_DEBUGSERVER_BUILD`. This replaces the old `LLDB_NO_DEBUGSERVER` flag. Doing the same for darwin-debug while I am here.

Reviewers: xiaobai, JDevlieghere, davide

Reviewed By: xiaobai, JDevlieghere

Subscribers: mgorny, lldb-commits, #lldb

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D64994

llvm-svn: 366631