Add option eTypeOptionHideEmptyAggregates.

Summary:
For certain data structures, when the synthetic child provider returns
zero children, a summary like "Empty instance of <typename>" could be
more appropriate than something like "size=0 {}". This new option helps
hide the trailing "{}".

This is also exposed with a -h option for the command "type summary add".

Reviewers: granata.enrico

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D11473

llvm-svn: 243166