A read access asserts that the memory location is immutable for the duration
of the access, so it can be treated as a borrow rather than a mutable lvalue.
Doing this allows the borrow formal access scope fixes from #79084 to apply
to situations where a loadable type undergoes an accessor-based access with
indirect arguments (such as for public accessors when library evolution is
enabled for the type). Fixes rdar://143334632.
