Self parameters & C-variadics should be syntactically legal in parenthesized generic argument lists

Self parameters shouldn't be syntactically forbidden in this position (only semantically) since that diverges from every other place that can contain "function parameters" IINM.

We intentionally allow self parameters in e.g., function pointer types (CC https://github.com/rust-lang/rust/pull/68764, https://github.com/rust-lang/reference/pull/2240#issuecomment-4320921175) and free function items. As it stands, it means we reject X(self), X(&self), X(&mut self) but accept X(&&self), X(&&mut self) for example (the latter are rightfully legal as they're interpreted as X(_: &&self) & X(_: &&mut self) essentially[^1]).

Similarly, for C-variadics. They should be syntactically allowed, too, IMO.

Unifying this would once again simplify the grammar; we shouldn't fight the fact that self parameters & C-variadics are (meant to be) syntactically valid fn params always.

[^1]: They're however always semantically ill-formed as self is a module not a type.

Originally posted by @fmease in https://github.com/rust-lang/rust/issues/158517#issuecomment-5230955069

No file or test is named in the issue. Start by locating the parser rules and existing tests for parenthesized generic argument lists, self parameters, and C-variadics; done means the listed forms are syntactically accepted while their existing semantic validity is preserved.