[CMake] Prevent lldbDebugserverCommon from building if you disable debugserver builds

Summary:
The flags `LLDB_USE_SYSTEM_DEBUGSERVER` and `LLDB_NO_DEBUGSERVER` were
introduced to the debugserver build. If one of these two flags are set, then we
do not build and sign debugserver. However I noticed that we were still building
the lldbDebugserverCommon and lldbDebugserverCommon_NonUI libraries regardless
of whether or not these flags were set. I don't believe we should be building
these libraries unless we are building and signing debugserver.

Reviewers: sgraenitz, davide, JDevlieghere, beanz, vsk, aprantl, labath

Subscribers: mgorny, jfb, lldb-commits

Differential Revision: https://reviews.llvm.org/D56763

llvm-svn: 351496