Auto merge of #123441 - saethlin:fixed-len-file-names, r=oli-obk

Stabilize the size of incr comp object file names

The current implementation does not produce stable-length paths, and we create the paths in a way that makes our allocation behavior is nondeterministic. I think `@eddyb` fixed a number of other cases like this in the past, and this PR fixes another one. Whether that actually matters I have no idea, but we still have bimodal behavior in rustc-perf and the non-uniformity in `find` and `ls` was bothering me.

I've also removed the truncation of the mangled CGU names. Before this PR incr comp paths look like this:
```
target/debug/incremental/scratch-38izrrq90cex7/s-gux6gz0ow8-1ph76gg-ewe1xj434l26w9up5bedsojpd/261xgo1oqnd90ry5.o
```
And after, they look like this:
```
target/debug/incremental/scratch-035omutqbfkbw/s-gux6borni0-16r3v1j-6n64tmwqzchtgqzwwim5amuga/55v2re42sztc8je9bva6g8ft3.o
```

On the one hand, I'm sure this will break some people's builds because they're on Windows and only a few bytes from the path length limit. But if we're that seriously worried about the length of our file names, I have some other ideas on how to make them smaller. And last time I deleted some hash truncations from the compiler, there was a huge drop in the number if incremental compilation ICEs that were reported: https://github.com/rust-lang/rust/pull/110367https://github.com/rust-lang/rust/pull/110367

---

Upon further reading, this PR actually fixes a bug. This comment says the CGU names are supposed to be a fixed-length hash, and before this PR they aren't: ca7d34efa9/compiler/rustc_monomorphize/src/partitioning.rs (L445-L448)

compiler/rustc_symbol_mangling/src/v0.rs

@@ -1,4 +1,4 @@
-use rustc_data_structures::base_n;
+use rustc_data_structures::base_n::ToBaseN;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::intern::Interned;
 use rustc_hir as hir;
@@ -832,7 +832,7 @@ impl<'tcx> Printer<'tcx> for SymbolMangler<'tcx> {
 ///   e.g. `1` becomes `"0_"`, `62` becomes `"Z_"`, etc.
 pub(crate) fn push_integer_62(x: u64, output: &mut String) {
     if let Some(x) = x.checked_sub(1) {
-        base_n::push_str(x as u128, base_n::ALPHANUMERIC_ONLY, output);
+        output.push_str(&x.to_base(62));
     }
     output.push('_');
 }