opt-viewer: fix HtmlFormatter encoding

Summary: Small fix to HtmlFormatter, defaults to ascii encoding, so utf-8 output may get `UnicodeEncodeError: 'ascii' codec can't encode character ... ordinal not in range(128)` during write.

Patch by Brian Cain!

Reviewers: anemet, fhahn

Reviewed By: anemet

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29802

llvm-svn: 294710