Thread Safety Analysis: Avoid infinite recursion in an operator<<

r217556 introduced an operator<<(std::ostream &, StringRef) that seems
to self recurse on some systems, because str.data(), which is a char *,
was being implicitly converted back to StringRef in overload
resolution.

This manifested as SemaCXX/warn-thread-safety-analysis.cpp timing out
in release builds and overflowing the stack in debug builds. One of
the failing systems that saw this is here:

  http://lab.llvm.org:8013/builders/clang-x86_64-darwin11-nobootstrap-RAincremental/builds/4636

Using ostream's write method instead of operator<< should get the bots
going again.