MCStreamer: Reimplement the virtual EmitRawText as a protected member, EmitRawTextImpl, to avoid string literal ambiguities

Also improve the implementation of EmitRawText(Twine) so it doesn't
bother using the SmallString buffer if the Twine is a simple StringRef
anyway.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193378 91177308-0d34-0410-b5e6-96231b3b80d8