[llvm] [ADT] Always use 32-bit size type for SmallVector with 16-bit elements (PR #95536)

[Jay Foad via llvm-commits]

jayfoad wrote:

> Please add some tests for common types like `char`, `int16_t`, etc. (e.g., using `std::is_same_v<T1, T2>`)

There were already some static_asserts for this kind of thing. I added one for int16_t.

https://github.com/llvm/llvm-project/pull/95536