Declare proto2::internal::Allocate always_inline and malloc

Clang provides the AllocToken feature [1], which enables transparently augmenting standard allocation API calls with type-derived tokens resulting in use of the alloc-token ABI, a different allocator ABI. This can be used for heap hardening strategies such as heap partitioning.

While for syntactically typed allocations (`new T`) the compiler can simply derive the token from `T`, for syntactically *untyped* allocations (`malloc(size)`, `::operator new(size)`), the compiler does best-effort analysis to determine the type from the size-expression.

Allocation wrappers of syntactically untyped allocations are a currently known gap, because the compiler cannot infer any allocated type from the inner standard allocation call.

`proto2::internal::Allocate()` is one such allocation wrapper.

To solve, Clang 23 is adding support to propagate inferred types from wrapper call-sites to the inner allocation when inlined [2].

For Clang to infer allocations through `proto2::internal::Allocate()`, add the `always_inline` and `malloc` attributes (the former ensures inlining, the latter tells the compiler it's an allocation function).

[1] https://clang.llvm.org/docs/AllocToken.html
[2] https://github.com/llvm/llvm-project/pull/198535

PiperOrigin-RevId: 922768652