Fix evaluating trivial drop glue in constants

[oli-obk]

struct A;
impl Drop for A {
    fn drop(&mut self) {}
}

const FOO: Option<A> = None;

const BAR: () = (FOO, ()).1;

was erroring with

error: any use of this value will cause an error
 --> src/lib.rs:9:1
  |
9 | const BAR: () = (FOO, ()).1;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^-^
  |                           |
  |                           calling non-const function `std::ptr::real_drop_in_place::<(std::option::Option<A>, ())> - shim(Some((std::option::Option<A>, ())))`
  |
  = note: #[deny(const_err)] on by default

error: aborting due to previous error

before this PR. According to godbolt this last compiled successfully in 1.27