Qualify anonymous enum references with their backing class for exclude-using-statics-for-enums

[tannergooding]

Fixes #681.

With exclude-using-statics-for-enums, a cross-context enum constant reference is emitted as EnumName.Member. For an anonymous enum there is no generated enum type — the members are lowered to const fields on the enum's backing class (Methods here) — so qualifying with the synthesized __AnonymousEnum_* name referenced a type that doesn't exist:

public enum MyEnum2
{
    MyEnum2_Value1 = __AnonymousEnum_header_L1_C1.MyEnum1_Value1,  // doesn't compile
}

VisitDeclRefExpr's DontUseUsingStaticsForEnums branch wrote enumName unconditionally. The using-statics branch already special-cases anonymous enums via GetClass(enumName); this mirrors that, so the reference now qualifies with the backing class:

public enum MyEnum2
{
    MyEnum2_Value1 = Methods.MyEnum1_Value1,
}

Verified live:

• anonymous enum + exclude-using-statics-for-enums → Methods.MyEnum1_Value1 (was __AnonymousEnum_*.)
• default (using-statics) path unchanged → using static ...Methods; + unqualified MyEnum1_Value1
• named cross-enum ref + exclude-using-statics-for-enums unchanged → MyEnum1.MyEnum1_Value1

Added AnonymousEnumReferenceExcludeUsingStaticsTest. Build 0 warning / 0 error; full suite green with no golden changes.

Note

This PR description and the code change were drafted by Copilot on my behalf.