[lldb-mi] Fix breakpoints on functions when C++ namespaces are used.

The command "-break-insert ns::foo" for function 'foo' in namespace 'ns'
was being parsed as file:function. This patch fixes these cases by adding
checks for '::'. (Note: '::func' is not parsed correctly by lldb due to
llvm.org/pr24271).

Reviewed by: ki.stfu
Subscribers: lldb-commits
Differential Revision: http://reviews.llvm.org/D11396