clang-format: Allow empty records on a single line

Feel free to close this if this is already possible and I just failed to figure out how.

There doesn't appear to be a way to accomplish this with clang-format:

```cpp
// Break before braces after class/struct/union:
struct Foo
{
int x;
};

// AND allow empty class/struct/union on a single line:
struct Foo {};
```

If we set `BraceWrapping: AfterStruct: true` and `BraceWrapping: SplitEmptyRecord: false`, we instead get this:

```cpp
struct Foo
{};
```

My proposal would be to add a `bool AllowEmptyRecordsOnASingleLine` option that handles this case similarly to the `AllowShortEnumsOnASingleLine` option. If others think this would be an acceptable enhancement, I can look into implementing it myself, but I thought I'd request feedback before I put in the work.

Start by locating clang-format's handling of BraceWrapping, SplitEmptyRecord, and AllowShortEnumsOnASingleLine. Check the existing formatting tests for records and empty constructs; done means non-empty records still break before braces while empty records format as `struct Foo {};`.

Written by the indexing model from the issue text.