Change the "bt" command alias defined in CommandInterpreter::LoadCommandDictionary.

It is now a regex command alias that more faithfully emulates gdb's
behavior, most importantly, "bt 5" will backtrace 5 frames of the
currently selected thread.  "bt all" still backtraces all threads
(unlike gdb) and for users who have learned to use "bt -c 5", that
form is still accepted.

llvm-svn: 165300