Add implementation of built-in function len() for those lldb containers with

unambiguous iteration support.  So that we could, for example:

    ...

    REGs = lldbutil.get_GPRs(frame)
    print "Number of general purpose registers: %d" % len(REGs)
    for reg in REGs:
        print "%s => %s" %(reg.GetName(), reg.GetValue())

    ...

llvm-svn: 131418