Fix -break-enable/-break-disable commands (MI)
- Previously -break-enable mistakenly set BP's enabled flag to false.
- These commands print fake =breakpoint-modified messages, what's not needed anymore because that events are come in normal way.
- Add tests for -break-enable/-break-disable commands
Initial patch from email@example.com. The test case was improved by me.
Differential Revision: http://reviews.llvm.org/D21757