<rdar://problem/13404189>

Made the "--reverse" option to "source list" also be able to use the "--count". This helps us implement support for regexp source list command:

(lldb) l -10

Which gets turned into:

(lldb) source list --reverse --count 10

Also simplified the code that is used to track showing more source from the last file and line.

llvm-svn: 176961