Auto merge of #45575 - michaelwoerister:tweak-inline-trans-items, r=nikomatsakis

Only instantiate inline- and const-fns if they are referenced (again).

It seems that we have regressed on not translating `#[inline]` functions unless they are actually used. This should bring back this optimization. I also added a regression test this time so it doesn't happen again accidentally.

Fixes #40392.

r? @alexcrichton

UPDATE & PSA
---------------------
This patch **makes translation very lazy** -- in general this is a good thing (we don't want the compiler to do unnecessary work) but it has two consequences:
1. Some error messages are only generated when an item is actually translated. Consequently, this patch will lead to more cases where the compiler will only start emitting errors when the erroneous function is actually used. This has always been true to some extend (e.g. when passing generic values to an intrinsic) but since this is something user-facing it's worth mentioning.
2. When writing tests, one has to make sure that the functions in question are actually generated. In other words, it must not be dead code. This can usually  be achieved by either
    1. making sure the function is exported from the resulting binary or
    2. by making sure the function is called from something that is exported (or `main()`).

Note that it depends on the crate type what functions are exported:
   1. For rlibs and dylibs everything that is reachable from the outside is exported.
   2. For executables, cdylibs, and staticlibs, items are only exported if they are additionally `#[no_mangle]` or have an `#[export_name]`.

The commits in this PR contain many examples of how tests can be updated to comply to the new requirements.