Fix cindex-dump.py --show-ids

This option flag was incorrectly expecting an argument:

$ cindex-dump.py --show-ids test.cpp
cindex-dump.py: error: invalid number arguments

With this change the feature correctly gets enabled by --show-ids.

No tests.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207946 91177308-0d34-0410-b5e6-96231b3b80d8