[clang] Fix MSVC template parsing error in SerializationFormat (PR #196571)

[Romaric Jodin via cfe-commits]

rjodinchr wrote:

> ~Thank you for the detailed explanation of this fix! Please update the code comments too.~ Looks like the change does not work?

I'm not able to reproduce on my machine, but I'm suspecting an issue regarding `llvm::function_ref` as it is a non-owning wrapper.

I still need to update the comments. I'll do that once all CI checks are green.

https://github.com/llvm/llvm-project/pull/196571