[CMake] Make liblldb depend on clang-headers

[xiaobai]

Summary

The clang headers are useful when dealing with clang modules. There is also a
way to get to the clang headers from the SB API so it would be nice if they were
also available when we just build lldb.

[teemperor]

LGTM.

source/API/CMakeLists.txt

if ( CMAKE_SYSTEM_NAME MATCHES "Windows" )
   endif()
 else()
   set_target_properties(liblldb
     PROPERTIES
     OUTPUT_NAME lldb
   )
 endif()
 
+if (NOT LLDB_BUILT_STANDALONE)
+  add_dependencies(liblldb clang-headers)
+endif()
+
 if(LLDB_BUILD_FRAMEWORK)
   include(LLDBFramework)
 endif()