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 xuefangliang@hotmail.com. The test case was improved by me.

Differential Revision: http://reviews.llvm.org/D21757