clang-format: Fix string literal breaking.

Before this patch, it did not cooperate with
Style::AlwaysBreakBeforeMultilineStrings. Thus, it would turn

  aaaaaaaaaaaa(aaaaaaaaaaaaa, "aaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaa");

into:

  aaaaaaaaaaaa(aaaaaaaaaaaaa, "aaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaa "
                              "aaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaa");

and only a second format step would lead to the desired (with that
option):

  aaaaaaaaaaaa(aaaaaaaaaaaaa,
               "aaaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaa "
               "aaaaaaaaaaaaaaaaaaaaa aaaaaaaaaaaaaaaaaaaaaa");

This could even lead to clang-format breaking the string at a different
character and thus leading to a completely different end result.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186154 91177308-0d34-0410-b5e6-96231b3b80d8