clang-format: Keep string-literal-label + value pairs on a line.

We have previously done that for <<-operators. This patch also adds
this logic for "," and "+".

Before:
  string v = "aaaaaaaaaaaaaaaa: " + aaaaaaaaaaaaaaaa + "aaaaaaaaaaaaaaaa: " +
             aaaaaaaaaaaaaaaa + "aaaaaaaaaaaaaaaa: " + aaaaaaaaaaaaaaaa;
  string v = StrCat("aaaaaaaaaaaaaaaa: ", aaaaaaaaaaaaaaaa, "aaaaaaaaaaaaaaaa: ",
                    aaaaaaaaaaaaaaaa, "aaaaaaaaaaaaaaaa: ", aaaaaaaaaaaaaaaa);

After:
  string v = "aaaaaaaaaaaaaaaa: " + aaaaaaaaaaaaaaaa +
	     "aaaaaaaaaaaaaaaa: " + aaaaaaaaaaaaaaaa +
	     "aaaaaaaaaaaaaaaa: " + aaaaaaaaaaaaaaaa;
  string v = StrCat("aaaaaaaaaaaaaaaa: ", aaaaaaaaaaaaaaaa,
		    "aaaaaaaaaaaaaaaa: ", aaaaaaaaaaaaaaaa,
		    "aaaaaaaaaaaaaaaa: ", aaaaaaaaaaaaaaaa);

llvm-svn: 289531