clang-format: Make AlwaysBreakBeforeMultilineStrings more conservative.

In essence this is meant to consistently indent multiline strings by a
fixed amount of spaces from the start of the line. Don't do this in
cases where it wouldn't help anyway.

Before:
  someFunction(aaaaa,
               "aaaaa"
               "bbbbb");

After:
  someFunction(aaaaa, "aaaaa"
                      "bbbbb");

llvm-svn: 240004