[lit] Parse all strings as UTF-8 rather than ASCII.

As far as I can tell UTF-8 has been supported since the beginning of Python's
codec support, and it's the de facto standard for text these days, at least
for primarily-English text. This allows us to put Unicode into lit RUN lines.

rdar://problem/18311663

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217688 91177308-0d34-0410-b5e6-96231b3b80d8