Add SB API's for writing breakpoints to & creating the from a file.

Moved the guts of the code from CommandObjectBreakpoint to Target (should
have done it that way in the first place.)  Added an SBBreakpointList class
so there's a way to specify which breakpoints to serialize and to report the
deserialized breakpoints.

<rdar://problem/12611863> 

llvm-svn: 281520