fix: remove non-conforming size-0 arrays

[eric-wieser]

In C, these are supported only as a vendor extension; they should instead use proper C99 flexible array members.

In C++, both [] and [0] are vendor extensions.

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase d0ee9d012774d4343ca2078b19adb11b10940fe0 --onto 5e8718dff9d7906e1d4ca7020256dae7c05e49c2. (2024-10-01 14:08:55)

[tkoeppe]

We wanted to note separately that the header lean.h, despite best efforts, is not actually conforming as a C and C++ interop header, since neither zero-sized nor flexible array members are conforming C++.

This change is still useful to make non-conforming C into conforming C, but as an interop header, the header should not contain these type definitions at all, and only declare opaque functions that can be used from both languages. The struct types with flexible array members should only exist in the C code.