[clang] [clang][serialization] Pass `ASTContext` explicitly (PR #115235)

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

To clarify, the main reason for this patch is that I find passing a reference much less error-prone than having an omnipresent pointer that will only be null during dependency scanning.

https://github.com/llvm/llvm-project/pull/115235