[cmake] Fix LLDB_CODESIGN_IDENTITY logic.

Summary:
Consolidate LLDB_CODESIGN_IDENTITY logic in one place and use
SKIP_DEBUGSERVER, which can be set independently, to control
codesigning targets.

Currently, running cmake the first time in a clean directory, without
passing -DLLDB_CODESIGN_IDENTITY='', fails.  However, subsequent runs
succeed.  That's because LLDB_CODESIGN_IDENTITY gets added to the
CACHE after the initial test.  To fix that, the default value must be
set before it's tested.

Here's the error produced on the first run:

CMake Error at tools/lldb/tools/debugserver/source/CMakeLists.txt:215 (add_custom_command):
  No TARGET 'debugserver' has been created in this directory.

Differential Revision: https://reviews.llvm.org/D43432

llvm-svn: 325442