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

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.)