When we want a runtime pointer to a zero-bit value we use an undef
pointer, but what if we want a runtime pointer to a comptime-only value?
Normally, if `T` is a comptime-only type such as `*const comptime_int`,
then `*const T` would also be a comptime-only type, so anything
referencing a comptime-only value is usually also comptime-only, and
therefore not emitted to the executable.

However, what if instead we have a `*const anyopaque` pointing to a
comptime-only value?  Certainly, `*const anyopaque` is a runtime type,
and so we need some runtime value to store, even when it happens to be
pointing to a comptime-only value.  In this case we want to do the same
thing as we do when pointing to a zero-bit value, so we use
`hasRuntimeBits` to handle both cases instead of ignoring comptime.

Closes #12025