Auto merge of #71232 - eddyb:print-const-adts, r=oli-obk

ty/print: pretty-print constant aggregates (arrays, tuples and ADTs).

Oddly enough, we don't have any UI tests showing this off in types, only `mir-opt` tests.
However, the pretty form should show up in the test output diff of #71018, if this PR is merged first.

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Examples of before/after:
|`Option<bool>`|
|:-:|
|`{transmute(0x01): std::option::Option<bool>}`|
| :sparkles: ↓↓↓ :sparkles: |
|`std::option::Option::<bool>::Some(true)`|

| `RawVec<u32>` |
|:-:|
| `ByRef { alloc: Allocation { bytes: [4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0], relocations: Relocations(SortedMap { data: [] }), undef_mask: UndefMask { blocks: [65535], len: Size { raw: 16 } }, size: Size { raw: 16 }, align: Align { pow2: 3 }, mutability: Not, extra: () }, offset: Size { raw: 0 } }: alloc::raw_vec::RawVec::<u32>`|
| :sparkles: ↓↓↓ :sparkles: |
|`alloc::raw_vec::RawVec::<u32> { ptr: std::ptr::Unique::<u32> { pointer: {0x4 as *const u32}, _marker: std::marker::PhantomData::<u32> }, cap: 0usize, alloc: std::alloc::Global }`|

<hr/>

This PR is a prerequisite for #61486, *sort of*, in that we need to be able to pretty-print values in order to even consider how we might mangle them.
We still don't have pretty-printing for constants of reference types, @oli-obk has the necessary support logic in a PR but I didn't want to interfere with that.

<hr/>

Each commit should be reviewed separately, as I've fixed a couple deficiencies along the way.

r? @oli-obk cc @rust-lang/wg-mir-opt @varkor @yodaldevoid

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