Auto merge of rust-lang#87194 - eddyb:const-value-mangling, r=michaelwoerister,oli-obk

rustc_symbol_mangling: support structural constants and &str in v0.

This PR should unblock rust-lang#85530 (except for float `const` generics, which AFAIK should've never worked).
(cc `@tmiasko` could the rust-lang#85530 (comment) failures be retried with a quick crater "subset" run of this PR + changing the default to `v0`? Just to make sure I didn't miss anything other than the floats)

The encoding is the one suggested before in e.g. rust-lang#61486 (comment), tho this PR won't by itself finish rust-lang#61486, before closing that we'd likely want to move to `@oli-obk's` "valtrees" (i.e. rust-lang#83234 and other associated work).

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**EDITs**:
1. switched unit/tuple/braced-with-named-fields `<const-fields>` prefixes from `"u"`/`"T"`/`""` to `"U"`/`"T"`/`"S"` to avoid the ambiguity reported by `@tmiasko` in rust-lang#87194 (comment).

2. `rustc-demangle` PR: rust-lang/rustc-demangle#55

3. RFC amendment PR: rust-lang/rfcs#3161
    * also removed the grammar changes included in that PR, from this description

4. added tests (temporarily using my fork of `rustc-demangle`)

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r? `@michaelwoerister`

Author: bors

std/Cargo.toml

@@ -24,7 +24,7 @@ std_detect = { path = "../stdarch/crates/std_detect", default-features = false,
 
 # Dependencies of the `backtrace` crate
 addr2line = { version = "0.16.0", optional = true, default-features = false }
-rustc-demangle = { version = "0.1.18", features = ['rustc-dep-of-std'] }
+rustc-demangle = { version = "0.1.21", features = ['rustc-dep-of-std'] }
 miniz_oxide = { version = "0.4.0", optional = true, default-features = false }
 [dependencies.object]
 version = "0.26.1"