[mojo-stdlib] Remove uses of `__get_lvalue_as_address`

[JoeLoser]

Review Mojo's priorities

• [X] I have read the roadmap and priorities and I believe this request falls within the priorities.

What is your request?

Replace uses of __get_lvalue_as_address so we can remove the builtin from the compiler itself. Everything using __get_lvalue_as_address(expr) can move to using Reference(expr) and Reference(expr).get_unsafe_pointer() should theoretically be a mechanical replacement.

What is your motivation for this change?

Doing this will allow us to remove the builtin from the compiler itself.

Any other details?

No response