Move SystemInitializerFull header to source/API

Summary:
It seems to me that files in include/lldb/API/ are headers that should
be exposed to liblldb users. Because SystemInitializerFull.h exposes details of
lldb_private, I think having it there is not the right thing to do. Since it's
only included from files in source/API, we should move it there and treat it as
private.

Reviewers: labath, clayborg

Reviewed By: labath, clayborg

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D47342

llvm-svn: 333304