clang-format: Properly align ObjC string literals.

clang-format: Properly align ObjC string literals.

Before:
  NSString s = @"a"
               "b"
               "c";
  NSString s = @"a"
               @"b"
                @"c";

After:
  NSString s = @"a"
                "b"
                "c";
  NSString s = @"a"
               @"b"
               @"c";

This fixes llvm.org/PR23536.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@237538 91177308-0d34-0410-b5e6-96231b3b80d8