Remove weak-linked symbols for SBBreakpointListImpl

Summary:
Similar to SBStructuredData's Impl class, SBBreakpointListImpl was
getting weak-link exported in the lldb namespace. This change list fixes
that by moving out of the lldb public namespace, which removes it from
public export visibility.

Fixes:
rdar://28960344

Reviewers: jingham

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D26553

llvm-svn: 286631