[cling] UNREACHABLE executed when unloading deduction guide

[hahnjo]

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Description

When built with assertions, Cling crashes when trying to unload deduction guides.

Reproducer

root [0] .rawInput
Using raw input
root [1] template <typename T> struct A { A(int) {} };
root [2] A(int) -> A<int>;
root [3] .rawInput
Not using raw input
root [4] .undo
Can't mangle a deduction guide name!
UNREACHABLE executed at /path/to/src/interpreter/llvm-project/clang/lib/AST/ItaniumMangle.cpp:1736!
Aborted (core dumped)

ROOT version

master, also reproduced with v6-32-00-patches, and probably happens with all ROOT versions built with C++17

Installation method

from source

Operating system

Linux

Additional context

Side remark: we may want to turn off LLVM_UNREACHABLE_OPTIMIZE, this would clearly error out for such cases in all configurations.