Output UTF-8-encoded characters as identifier characters into assembly

Output UTF-8-encoded characters as identifier characters into assembly
by default.

This is a behaviour configurable in the MCAsmInfo. I've decided to turn
it on by default in (possibly optimistic) hopes that most assemblers are
reasonably sane. If this proves a problem, switching to default seems
reasonable.

I'm not sure if this is the opportune place to test, but it seemed good
to make sure it was tested somewhere.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@154235 91177308-0d34-0410-b5e6-96231b3b80d8