Fix #11004: preserve D-style variadics in SetFunctionAttributes

[AayushMainali-Github]

Rationale

SetFunctionAttributes rebuilt D-style variadic function types by appending "..." after Parameters!T with no comma. For a function like void fn(int a, ...), that produced void function(int...) (typesafe variadic) instead of void function(int, ...) (D-style variadic).

The mixin now emits ", ..." when there are fixed parameters, and "..." when there are none. Regression coverage was added for D-style variadics with and without fixed parameters, and typesafe variadics are still preserved.

Verified locally with make -f posix.mak BUILD=debug std/traits.test (1 modules passed unittests), and with a direct compile of the issue repro (fails before the fix, passes after).

Feature request or issue tracking

Closes #11004.

Pre-review checklist

• I have performed a self-review of my code.
• If my PR fixes a bug or introduces a new feature, I have added thorough tests.
• If my changes are non-trivial and do not concern a reported issue, I have added a changelog entry.

LLM/AI disclosure

This PR was written with the assistance of an LLM/AI.