[lldb-mi] display summary for simple types + refactor (use lldb formatting for all cases)

Previously, lldb did not use type summaries for simple types with no children
(like function pointers).  This patch enables MI to use lldb type summaries for
evaluation of all types of objects, so MI own formatters are no longer needed.

Patch from evgeny.leviant@gmail.com
Reviewed by: abidh
Subscribers: lldb-commits
Differential Revision: http://reviews.llvm.org/D13799

llvm-svn: 251082