swift234

Create an entrypoint tryToConcatenateStrings and hide the class StringConcatenationOptimizer in Local.cpp.

In every instance, we were just creating the StringConcatenationOptimizer and
then invoking optimize on it. This is a cleaner solution since the details of
how we perform the string concatenation are hidden in Local.cpp instead of being
in a header.

NFC.

Swift SVN r25341