Avoid binding a reference to a dereferenced pointer in diagnostic serialization

Avoid binding a reference to a dereferenced pointer in diagnostic serialization
if a diagnostic is emitted outside of any source file. The fix mirrors the
corresponding code in TextDiagnosticPrinter. This required moving the
functional parts of SDiagRenderer into SDiagWriter so they can be reused in the
non-rendering codepath.

No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@162253 91177308-0d34-0410-b5e6-96231b3b80d8