Rollup merge of #92316 - petrochenkov:extmangle, r=wesleywiser

mangling_v0: Skip extern blocks during mangling

There's no need to include the dummy `Nt` into the symbol name, items in extern blocks belong to their parent modules for all purposes except for inheriting the ABI and attributes.

Follow up to https://github.com/rust-lang/rust/pull/92032

(There's also a drive-by fix to the `rust-demangler` tool's tests, which don't run on CI, I initially attempted using them for testing this PR.)

compiler/rustc_symbol_mangling/src/v0.rs

@@ -772,9 +772,9 @@ impl<'tcx> Printer<'tcx> for &mut SymbolMangler<'tcx> {
         disambiguated_data: &DisambiguatedDefPathData,
     ) -> Result<Self::Path, Self::Error> {
         let ns = match disambiguated_data.data {
-            // FIXME: It shouldn't be necessary to add anything for extern block segments,
-            // but we add 't' for backward compatibility.
-            DefPathData::ForeignMod => 't',
+            // Extern block segments can be skipped, names from extern blocks
+            // are effectively living in their parent modules.
+            DefPathData::ForeignMod => return print_prefix(self),
 
             // Uppercase categories are more stable than lowercase ones.
             DefPathData::TypeNs(_) => 't',