[clang] [serialization] no transitive decl change (PR #92083)

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

I only managed to confirm that the problem is that merging of individual PCMs into a merged hash table has become much slower with this change.
I tried with #95348 too, but it didn't help.

I will continue my investigations and reductions tomorrow.

https://github.com/llvm/llvm-project/pull/92083