[CMake] Refactor LLDB main CMake file.

The existing state of affairs was getting a little unwieldy.

All of LLDB's utility functions and initial configuration was in
the root CMake file.  I split this up into 3 separate files and
moved them to relevant subfolders under cmake/modules.

Also, I deleted the add_lldb_definitions() function.  It seemed
to be somewhat useless and did not serve any real purpose that
I was able to figure out.

llvm-svn: 231010