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

utils/opt-viewer/opt-viewer.py

@@ -183,7 +183,7 @@ def __init__(self, source_dir, output_dir, filename):
 </html>
             '''.format(filename), file=self.stream)
 
-        self.html_formatter = HtmlFormatter()
+        self.html_formatter = HtmlFormatter(encoding='utf-8')
         self.cpp_lexer = CppLexer()
 
     def render_source_line(self, linenum, line):