[clang] [clang] Fix ASTWriter crash after merging named enums (PR #114240)

[Michael Jabbour via cfe-commits]

michael-jabbour-sonarsource wrote:

Gentle ping :smile:

The aim of the PR is to fix an ASTWriter crash that currently happens when serializing merged enums. Such a case was encountered while parsing Objective-C code that uses the XCode 16 SDK. If any of the changes look risky, I am happy to rework them and/or add more tests to increase confidence.

Thanks in advance.

https://github.com/llvm/llvm-project/pull/114240