[clang] [Clang] [NFC] Prevent null pointer dereference in Sema::InstantiateFunctionDefinition (PR #89801)

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https://github.com/Sirraide commented:

A pr that just adds assertions and nothing else seems like a very weird thing to do. Moreover, a few lines above in the same function, we return if this isn’t a copy/move ctor/assignment operator, which always has at least one parameter; I don’t think it is possible for `Parm` to be `nullptr` here. Do you have any code examples that would trigger this asertion here?

https://github.com/llvm/llvm-project/pull/89801