Check in an initial implementation of the "breakpoint clear" command, whose purpose is clear

the breakpoint associated with the (filename, line_number) combo when an arrow is pointing to
a source position using Emacs Grand Unified Debugger library to interact with lldb.

The current implmentation is insufficient in that it only asks the breakpoint whether it is
associated with a breakpoint resolver with FileLine type and whether it matches the (filename, line_number)
combo.  There are other breakpoint resolver types whose breakpoint locations can potentially
match the (filename, line_number) combo.

The BreakpointResolver, BreakpointResolverName, BreakpointResolverAddress, and BreakpointResolverFileLine
classes have extra static classof methods to support LLVM style type inquiry through isa, cast, and dyn_cast.

The Breakpoint class has an API method bool GetMatchingFileLine(...) which is invoked from CommandObjectBreak.cpp
to implement the "breakpoint clear" command.

llvm-svn: 117562