Improve line breaking before multi-line strings.

The AlwaysBreakBeforeMultilineStrings rule does not really make sense
if it does not a column gain.

Before (in Google style):
  f(
      "aaaa"
      "bbbb");

After:
  f("aaaa"
    "bbbb");

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186515 91177308-0d34-0410-b5e6-96231b3b80d8