[LLDB] Fix/silence CMake developer warning for LLDB framework.

This fixes the following warning for developers:

  Target 'liblldb' was changed to a FRAMEWORK sometime after install().  This
  may result in the wrong install DESTINATION.  Set the FRAMEWORK property
  earlier.

The solution is to pass the FRAMEWORK flag to add_lldb_library and set
the target property before install(). For now liblldb is the only
customer.