clang-format: [Java] Understand string literal concatenation.

Before:
  String someString = "abc" + "cde";

After:
  String someString = "abc"
                      + "cde";

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220287 91177308-0d34-0410-b5e6-96231b3b80d8