Fix python3 compability issue in clang binding

The file contents could be of str type. Should use byte length instead
of str length, otherwise utf-8 encoded files may not get properly parsed
for completion.

Source issue: https://github.com/ncm2/ncm2-pyclang#2

Commited on behalf of `roxma'

Differential Revision: https://reviews.llvm.org/D56429

llvm-svn: 352039