[CMake] Streamline code signing for debugserver #2

Summary:
Major fixes after D54476 (use Diff1 as base for comparison to see only recent changes):
* In standalone builds target directory for debugserver must be LLDB's bin, not LLVM's bin
* Default identity for code signing must not force-override LLVM_CODESIGNING_IDENTITY globally

We have a lot of cases, make them explicit:

* ID used for code signing (debugserver and in tests):
** `LLDB_CODESIGN_IDENTITY` if set explicitly, or otherwise
** `LLVM_CODESIGNING_IDENTITY` if set explicitly, or otherwise
** `lldb_codesign` as the default

* On Darwin we have a debugserver target that:

* On other systems, the debugserver target is not defined, which is equivalent to **[3A]**

Common configurations on Darwin:
* **[1A]** `cmake -GNinja ../llvm` builds debugserver from source and signs with `lldb_codesign`, no code signing for other binaries (prints status: //lldb debugserver: /path/to/bin/debugserver//)
* **[1A]** `cmake -GNinja -DLLVM_CODESIGNING_IDENTITY=- -DLLDB_CODESIGN_IDENTITY=lldb_codesign ../llvm` builds debugserver from source and signs with `lldb_codesign`, ad-hoc code signing for other binaries (prints status: //lldb debugserver: /path/to/bin/debugserver//)
* **[2A]** `cmake -GNinja -DLLVM_CODESIGNING_IDENTITY=- -DLLDB_USE_SYSTEM_DEBUGSERVER=ON ../llvm` copies debugserver from system, ad-hoc code signing for other binaries (prints status: //Copy system debugserver from: /path/to/system/debugserver//)
* **[2B]** `cmake -GNinja -DLLVM_CODESIGNING_IDENTITY=- ../llvm` same, but prints additional warning: //Cannot code sign debugserver with identity '-'. Will fall back to system's debugserver. Pass -DLLDB_CODESIGN_IDENTITY=lldb_codesign to override the LLVM value for debugserver.//
* **[3A]** `cmake -GNinja -DLLVM_CODESIGNING_IDENTITY=- -DLLDB_NO_DEBUGSERVER=ON ../llvm` debugserver not available (prints status: //lldb debugserver will not be available)//

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

Reviewed By: JDevlieghere, labath

Subscribers: mgorny, #lldb, lldb-commits

Differential Revision: https://reviews.llvm.org/D55013

llvm-svn: 350388