Add FuncOp::eraseArgument

This is a quite complex operation that users are likely to attempt to write
themselves and get wrong (citation: users=me).

Ideally, we could pull this into FunctionLike, but for now, the
FunctionType rewriting makes it FuncOp specific. We would need some hook
for rewriting the function type (which for LLVM's func op, would need to
rewrite the underlying LLVM type).

PiperOrigin-RevId: 280234164