Improve formatting of stream operators.

Improve formatting of stream operators.

If there are string literals on either side of a '<<', chances are
high that they represent logically separate concepts. Otherwise,
the author could just have just a single literal (possible split
over multiple lines).

So, we can now nicely format things like:
cout << "somepacket = {\n"
     << "  val a = " << ValueA << "\n"
     << "  val b = " << ValueB << "\n"
     << "}";

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@174310 91177308-0d34-0410-b5e6-96231b3b80d8