Rewrite splitLines using StringRef; this is much simpler, and also now avoids

tons of std::string trashing. I plan to move this and other fun string munging
utilities to a StringRefExtras.h at some point if no one beats me to it.

On a synthetic benchmark on x86_64, llvm-gcc actually generates code thats 10%
faster using the StringRef version. gcc miscompiles the synthetic benchmark,
which I'm crossing my fingers and hoping won't happen here. clang compiles the
sythetic benchmark correctly (wootness), but the StringRef version is
slower. Silly clang.

llvm-svn: 86799