[clang-format] Use utf-8 for JSON object load

From Python 3.6 and above, it should be able to automatically select a
decoding for json.loads. However, with a vim encoding that defaults
to utf-8, clang-format.py runs into the following error

    TypeError: the JSON object must be str, not 'bytes'

This patch explicitly specifies utf-8 decoding for the header.

Reviewed by: ldrumm, sammcall

Differential Revision: https://reviews.llvm.org/D133236