[clang-format] Multi-line macro doesn't respect `ColumnLimit: 0`

I noticed that multi-line macros don't respect the `ColumnLimit: 0` setting.

So the following:

```C
#define PACK_REFS_OPTS(spaces) \
"[--all] [--no-prune] [--auto] [--include ] [--exclude ]\n" \
spaces "[--required]"
```

is always requested to be changed like:

```diff
#define PACK_REFS_OPTS_SPACES_14 " "
#define PACK_REFS_OPTS_SPACES_18 " "
-#define PACK_REFS_OPTS(spaces) \
- "[--all] [--no-prune] [--auto] [--include ] [--exclude ]\n" \
- spaces "[--required]"
+#define PACK_REFS_OPTS(spaces) \
+ "[--all] [--no-prune] [--auto] [--include ] [--exclude " \
+ "]\n" spaces "[--required]"
```

Start by reproducing the shown multi-line macro with clang-format and ColumnLimit: 0, then trace the formatting path for macro contents. Done means clang-format preserves the macro's existing line layout when that setting disables the column limit.

Written by the indexing model from the issue text.