[clang] [Clang] Look through type sugar when accessing FunctionProtoType (PR #88428)

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Sirraide wrote:

> Ooof, yeah, good catch. I should have caught that in the review, I've fallen for that one before :)

I was just now made aware of the fact that this asserts and the only reason I found the problem fairly quickly is because I happen to have a pr open at the moment that deals w/ type sugar on lambdas.

https://github.com/llvm/llvm-project/pull/88428