Add fast path for raw_ostream output of strings.

 - Particularly nice for small constant strings, which get optimized
   down nicely. On a synthetic benchmark writing out "hello" in a
   loop, this is about 2x faster with gcc and 3x faster with
   llvm-gcc. llc on insn-attrtab.bc from 403.gcc is about .5% faster.

 - I tried for a fancier solution which wouldn't increase code size as
   much (by trying to match constant arrays), but can't quite make it
   fly.

llvm-svn: 68396