[CMake] Update CMAKE_OSX_DEPLOYMENT_TARGET to 10.13.

The greendragon standalone bot is complaining about the deployment
target needing to be 10.13 or above. This change makes it 10.13

Author: rastogishubham

lldb/cmake/caches/Apple-lldb-Xcode.cmake

@@ -1,7 +1,7 @@
 include(${CMAKE_CURRENT_LIST_DIR}/Apple-lldb-base.cmake)
 
 set(CMAKE_GENERATOR Xcode CACHE STRING "")
-set(CMAKE_OSX_DEPLOYMENT_TARGET 10.12 CACHE STRING "")
+set(CMAKE_OSX_DEPLOYMENT_TARGET 10.13 CACHE STRING "")
 set(CMAKE_XCODE_GENERATE_SCHEME ON CACHE BOOL "")
 
 set(LLDB_BUILD_FRAMEWORK ON CACHE BOOL "")