The existing v0 tests have been slightly adjusted for compatibility with
legacy mangler, which requires an item to have an ancestor in a value
namespace or a type namespace to produce a symbol for it. In v0 mangling
this results in an extra `Nv` component.

The `PrettyPrinter` changes formatting of array size and integer
constants based on `-Zverbose`, so its implementation cannot be used in
legacy symbol mangling.

Continuation of rust-lang#94376.

cc rust-lang#53667

…rochenkov

Fix inconsistent symbol mangling of integers constants with -Zverbose

The `PrettyPrinter` changes formatting of array size and integer
constants based on `-Zverbose`, so its implementation cannot be used in
legacy symbol mangling.

Example symbol demangling before changes:

```console
$ cat a.rs
pub struct A<T>(T);
impl A<[u8; 128]> { pub fn f() {} }
$ rustc --crate-type=lib a.rs -Zverbose=n && nm -C ./liba.rlib
00000000 T a::A<[u8; 128]>::f
$ rustc --crate-type=lib a.rs -Zverbose=y && nm -C ./liba.rlib
00000000 T a::A<[u8; Const { ty. usize, val. Value(Scalar(0x0000000000000080)) }]>::f
```

6 - Make more use of `let_chains`

Continuation of rust-lang#94376.

cc rust-lang#53667

…=lnicola

⬆️ rust-analyzer

r? ``@ghost``