[clang-format] respect AfterEnum for enums

There is some similar looking code in `TokenAnnotator.cpp` but given that I've
never worked on clang-format before I don't know what the purpose of that code
is and how it's related to `UnwrappedLineParser.cpp`.

Either way, it fixes clang-format with `BraceWrapping.AfterEnum=true` and
`AllowShortEnumsOnASingleLine=false` to behave like the documentation says.

Before this patch:
```
enum
{
  A,
  B
} myEnum;
```

After this patch:
```
enum {
  A,
  B
} myEnum;
```

According to the unittests which I had to modify this would change the LLVM
style. Please evaluate if you want to change the defaults or if you consider
the current style a bug.

Reviewed By: curdeius, HazardyKnusperkeks

Differential Revision: https://reviews.llvm.org/D106349