[cmake] Copy system debugserver from the right place when only CommandLineTools are installed

[xiaobai]

Summary

Instead of building debugserver when building lldb, I'd rather pass
LLDB_CODESIGN_IDENTITY="" to cmake and use the one already on my system.
However, on one of my machines I only have the CommandLineTools installed, and
so the hardcoded path to the system debugserver does not work for me.
Additionally, we should verify the LLDB framework exists on the machine before
trying to set the path to debugserver. This allows us to warn the user at
configure time that a system debugserver can't be found if they choose not to
build it themselves.

[vsk]

Thank you, lgtm.

[xiaobai]

@vsk Would you mind committing this for me when you get the chance? I don't have commit access. :)

tools/debugserver/source/CMakeLists.txt

 if(NOT LLDB_CODESIGN_IDENTITY STREQUAL "")
   set(DEBUGSERVER_PATH $<TARGET_FILE:debugserver> CACHE PATH "Path to debugserver.")
   set(SKIP_DEBUGSERVER OFF CACHE BOOL "Skip building the in-tree debug server")
 else()
   execute_process(
     COMMAND xcode-select -p
     OUTPUT_VARIABLE XCODE_DEV_DIR)
   string(STRIP ${XCODE_DEV_DIR} XCODE_DEV_DIR)
+  if(EXISTS "${XCODE_DEV_DIR}/../SharedFrameworks/LLDB.framework/")
     set(DEBUGSERVER_PATH
       "${XCODE_DEV_DIR}/../SharedFrameworks/LLDB.framework/Resources/debugserver" CACHE PATH "Path to debugserver.")
+  elseif(EXISTS "${XCODE_DEV_DIR}/Library/PrivateFrameworks/LLDB.framework/")
+    set(DEBUGSERVER_PATH
+      "${XCODE_DEV_DIR}/Library/PrivateFrameworks/LLDB.framework/Resources/debugserver" CACHE PATH "Path to debugserver.")
+  else()
+    message(SEND_ERROR "Cannot find debugserver on system.")
+  endif()
   set(SKIP_DEBUGSERVER ON CACHE BOOL "Skip building the in-tree debug server")
 endif()
 message(STATUS "Path to the lldb debugserver: ${DEBUGSERVER_PATH}")
 
 if (APPLE)
   if(IOS)
     find_library(BACKBOARD_LIBRARY BackBoardServices
       PATHS ${CMAKE_OSX_SYSROOT}/System/Library/PrivateFrameworks)