Fix ICE in v0 symbol mangling for deeply nested generic types

[GokhanKabar]

Fixes #154965

When using -C symbol-mangling-version=v0, pathologically nested generic
type aliases cause the v0 mangler to emit a symbol so deeply nested that
rustc_demangle exceeds its MAX_DEPTH = 500 recursion limit and returns
Err. This trips the debug_assert! in compute_symbol_name, producing an ICE.

The fix mirrors the existing fallback for legacy mangling (which falls back
to v0 when symbols are too long for PDB): if the v0-mangled symbol cannot
be demangled, we fall back to hashed mangling, which always produces a
short, demanglable symbol.

A build-pass regression test is included.

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[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.