iwyu fixes for lldbCore.

This adjusts header file includes for headers and source files
in Core.  In doing so, one dependency cycle is eliminated
because all the includes from Core to that project were dead
includes anyway.  In places where some files in other projects
were only compiling due to a transitive include from another
header, fixups have been made so that those files also include
the header they need.  Tested on Windows and Linux, and plan
to address failures on OSX and FreeBSD after watching the
bots.

llvm-svn: 299714