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

[Michael Jabbour via cfe-commits]

michael-jabbour-sonarsource wrote:

Ping?

Note that this PR fixes a crash in ASTWriter on valid Objective-C modules code; The initial PR title didn't accurately reflect that.

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