[clang] [Clang][NFCI] Cleanup the fix for default function argument substitution (PR #104911)

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cor3ntin wrote:

Can you keep the tests as is? Otherwise it's hard to trust this is really NFC.
I have some concerns over the number of booleans we are adding to these functions, did you consider an enum or a less easy to miss use interface?

https://github.com/llvm/llvm-project/pull/104911