clang-format: Don't force linebreak between return and multiline string.

This looks ugly and leads to llvm.org/PR17590.

Before (with AlwaysBreakBeforeMultilineStrings):
  return
      "aaaa"
      "bbbb";

After:
  return "aaaa"
         "bbbb";

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192984 91177308-0d34-0410-b5e6-96231b3b80d8