Resubmit "[CMake] Change lldbAPI to be a CMake OBJECT library."

This resubmits r230380.  The primary cause of the failure was
actually just a warning, which we can disable at the CMake level
in a followup patch on the LLVM side.  The other thing which was
actually an error on the bot should be able to be fixed with
a clean.

llvm-svn: 230389