cindex/Python: Fix cindex-{dump,includes} examples to just pass all args

directly to Index, instead of requiring the input file to be first. This makes
the examples behave more like 'clang'.

For example,
  ddunbar@giles:tmp$ echo '#include <string>' | python ~/llvm/tools/clang/bindings/python/examples/cindex/cindex-includes.py -- -x c++ - | wc -l
     114
  ddunbar@giles:tmp$ echo '#include <stdio.h>' | python ~/llvm/tools/clang/bindings/python/examples/cindex/cindex-includes.py -- -x c - | wc -l
      10

llvm-svn: 96107