Co-authored-by: Nlcnr <rust@lcnr.de>

When encountering code like

```rust
fn foo() -> i32 {
    match 0 {
        1 => return 0,
        2 => "",
        _ => 1,
    }
}
```

Point at the return type and not at the prior arm, as that arm has type
`!` which isn't influencing the arm corresponding to arm `2`.

Fix #78124.

Co-authored-by: Nklensy <klensy@users.noreply.github.com>

Co-authored-by: Nbjorn3 <17426603+bjorn3@users.noreply.github.com>

It is only used by CrateLoader. We can store the metadata loader in
CStore instead which CrateLoader has access to.

Constructing an FxIndexMap is useless work as the iteration order never
matters.

All of them are not exported from rustc_interface and used only during
global_ctxt(). Inlining them makes it easier to follow the order of
queries and slightly reduces line count.

It was only ever set in a function which isn't called anywhere.

This allows for better type safety in the compiler and also improves the
documentation for many things, making it clear that they expect modules.

Storing coverage counter information in `CoverageCounters` has a few advantages
over storing it directly inside BCB graph nodes:

- The graph doesn't need to be mutable when making the counters, making it
easier to see that the graph itself is not modified during this step.

- All of the counter data is clearly visible in one place.

- It becomes possible to use a representation that doesn't correspond 1:1 to
graph nodes, e.g. storing all the edge counters in a single hashmap instead of
several.

This avoids the need to pass around a separate vector to accumulate into, and
avoids the need to create a fake empty vector when failure occurs.

This avoids confusion with data structures that actually hold BCB counter
information.