[clang-format] Regression: enumerator initializer continuation loses indent when wrapped (23 vs 22)

## Summary

In clang-format 23, when an enumerator's initializer is too long and must wrap
at a binary operator, the continuation line is emitted with no continuation
indentation — it is indented only to the enumerator's own level. In
clang-format 22 (and earlier) the continuation was correctly indented.

This only happens inside an `enum` body. The equivalent expression as a
namespace-scope or class-scope variable initializer still wraps correctly, so
the regression is specific to enumerator initializers.

## Steps to reproduce

Input (`enum_repro.cc`):

```c++
enum {
MyEnum = aaaaaaaaaaaaaaa + bbbbbbbbbbbbbbb
};
```

Command:

```
clang-format --style='{BasedOnStyle: LLVM, ColumnLimit: 40}' enum_repro.cc
```

## Actual vs expected

**clang-format 22.1.0-rc1 (expected):**

```c++
enum {
MyEnum =
aaaaaaaaaaaaaaa + bbbbbbbbbbbbbbb
};
```

**clang-format 23.1.0-rc1 (actual — buggy):**

```c++
enum {
MyEnum = aaaaaaaaaaaaaaa +
bbbbbbbbbbbbbbb
};
```

Start with enum_repro.cc and run clang-format with the LLVM style and ColumnLimit 40, comparing the output with the expected clang-format 22 result. Trace the clang-format formatting path for enum initializers and add regression coverage for the wrapped binary expression; done when the continuation is indented as expected without changing namespace- or class-scope initializer behavior.

Written by the indexing model from the issue text.