A bit more polish on const eval errors

[lqd]

This PR adds a bit more polish to the const eval errors:

• a slight improvement to the PME messages from Post-monomorphization errors traces MVP #85633: I mentioned there that the erroneous item's paths were dependent on the environment, and could be displayed fully qualified or not. This can obscure the items when they come from a dependency. This PR uses the pretty-printing code ensuring the items' paths are not trimmed.
• whenever there are generics involved in an item where const evaluation errors out, the error message now displays the instance and its const arguments, so that we can see which instantiated item and compile-time values lead to the error.

So we get this slight improvement for our beloved stdarch example, on nightly:

error[E0080]: evaluation of constant value failed
 --> ./stdarch/crates/core_arch/src/macros.rs:8:9
  |
8 |         assert!(IMM >= MIN && IMM <= MAX, "IMM value not in expected range");
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the evaluated program panicked at 'IMM value not in expected range', /rustc/9111b8ae9793f18179a1336417618fc07a9cac85/library/core/src/../../stdarch/crates/core_arch/src/macros.rs:8:9
  |

to this PR's:

error[E0080]: evaluation of `core::core_arch::macros::ValidateConstImm::<51_i32, 0_i32, 15_i32>::VALID` failed
 --> ./stdarch/crates/core_arch/src/macros.rs:8:9
  |
8 |         assert!(IMM >= MIN && IMM <= MAX, "IMM value not in expected range");
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the evaluated program panicked at 'IMM value not in expected range', ./stdarch/crates/core_arch/src/macros.rs:8:9
  |

with this PR.

Of course this is an idea from Oli, so maybe r? @oli-obk if they have the time.

[oli-obk]

cc @rust-lang/wg-const-eval

[oli-obk]

we already report such paths (with {constant#0}) in other situations. It's not ideal, but maybe better than not mentioning the path?

[lqd]

For reference, there are indeed a few cases in the ui tests where this happens today, for example

rust/src/test/ui/mir/issue-80742.stderr

Lines 10 to 14 in ce0d64e

	::: $DIR/issue-80742.rs:23:10
	|
	LL | [u8; size_of::<T>() + 1]: ,
	| -------------- inside `Inline::<dyn Debug>::{constant#0}` at $DIR/issue-80742.rs:23:10

(After/if this PR is merged, I'll work on changing these messages: to keep the originating span, but not the full instance path, so that they are not duplicated)

[oli-obk]

@bors r+

[bors]

📌 Commit c31ca9a has been approved by oli-obk

[bors]

⌛ Testing commit c31ca9a with merge 9f75dbf...

[bors]

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing 9f75dbf to master...