Casey's accumulated miscellaneous changes

[CaseyCarter]

A grab bag of minor changes broken down for quick-and-easy commit-wise review:

• <ranges>: simplify zip_view::_Size_closure by making it a static constexpr data member instead of a function
• Use conventional LLVM issue linking style
• Simplify _Tuple_like metaprogramming: implements _Tuple_like_impl as a variable template with four partial specializations instead of as a disjunction of four other variable template specializations. Consequently, each evaluation of _Tuple_like causes the compiler to memoize one constexpr variable instead of five.
• Don't return a decayed boolean-testable since the exposition-only concept doesn't allow for construction of an object of decayed type from a model of boolean-testable. Note that the changes in erase_if(forward_list, pred) and erase_if(list, pred) are the proposed resolution of LWG-4135, which I feel is a no-brainer and can be implemented without comments. I audited for other occurrences after noticing we do the same thing in forward_list::remove and list::remove.
  The fold expressions in <ranges> are a weird case: they are only problematic when the pack has one element. For zero or more than one elements, the fold expression has type bool, but with exactly one element it's a boolean-testable that the lambda implicitly decays on return. I think it's a bit more elegant to fix these by adding || false to the fold expressions to make them binary than to explicitly specify the return type of the lambda.
• Don't return in cartesian_product_view::_Iterator::iter_swap. We can return an expression with void type in a function with return type void, but we typically don't do so when the types are concrete.

I could be convinced that the boolean-testable change deserves to be in a separate PR with some test coverage.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Yay various cleanups! 🧹 🎉 🐈‍⬛