[lldb][NFC] Remove stride parameter from GetArrayElementType

This parameter isn't used anywhere in LLDB nor the Swift downstream branch. It
also doesn't really fit into the TypeSystem APIs that usually don't return
additional related functionality via some output parameters. Also the
implementations already states that the calculated value there is wrong.

Let's remove it. If we need this functionality at some point then Swift's much
nicer `GetByteStride` function seems like the way to go.

Reviewed By: aprantl

Differential Revision: https://reviews.llvm.org/D84299