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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@294710 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed