Expose ThreadCollection in SB API

Reviewed at
http://reviews.llvm.org/D5218
and
http://lists.cs.uiuc.edu/pipermail/lldb-commits/Week-of-Mon-20140901/012828.html



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@217296 91177308-0d34-0410-b5e6-96231b3b80d8