[clang] [Clang][Parser] Fix assertion failure with explicit(bool) in pre-C++20 modes (PR #152896)

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https://github.com/Sirraide requested changes to this pull request.

Er, unfortunately this is definitely not right: there’s a lot of unnecessary code duplication here, and the feature worked just fine up to and including Clang 20 (even in C++98 mode), and we definitely want to keep supporting it.

I wonder if we should just drop the assertion that causes this entirely.

https://github.com/llvm/llvm-project/pull/152896