[ItaniumDemangle] Strip __alloc_token_ to transparently demangle allocation functions (#191048)

Update the Itanium demangler to recognize and strip `__alloc_token_`
prefixes introduced by AllocToken instrumentation [1]. This ensures that
instrumented allocation functions (e.g., `__alloc_token__Znwm`) demangle
back to their original source-level names (e.g., `operator new(unsigned
long)`) with a suffix `(.alloc_token)` indicating their non-standard
origin.

Since AllocToken is intended to be transparent to users who continue to
use `operator new` as before, the demangled name should reflect this
reality instead of confusing users with internal instrumentation names
in stack traces or symbolization output.

Synchronize changes across llvm and libcxxabi copies.

[1] https://clang.llvm.org/docs/AllocToken.html

Original RFC: https://discourse.llvm.org/t/rfc-a-framework-for-allocator-partitioning-hints/87434

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GitOrigin-RevId: fb6f1bde00c4ed3650bd1152607718a95ca0ce6d