[clang] [clang][modules] Fix use-after-free in header serialization (PR #96356)

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

> My main concern with this approach is I'm not sure that after fixing the memory issue we don't have a remaining logic bug. Because we are still modifying `ModuleMap::Headers` during `ASTWriter::WriteHeaderSearch` iteration and that looks suspicious. Need to think how to make sure the discovered unexpected modification isn't causing other problems.

How do you suggest we verify this? I don't see Clang storing reference to the `KnownHeader` vector anywhere else.

https://github.com/llvm/llvm-project/pull/96356