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

[Volodymyr Sapsai via cfe-commits]

vsapsai wrote:

In general, personally I quite like the idea of removing decls from the scope. Though I think when I've tried to do so [in a different context], there were some problems. I'll try to find what I was doing and why it wasn't working. It's not a blocker for your change but it can be helpful to understand potential pitfalls better.

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