[clang-cl] Crash in MSVC ABI mangler when mangling template function with [[clang::annotate_type]] and deduced return type

Clang crashes (segfault) in the MSVC ABI mangler when mangling a template function that has [[clang::annotate_type(...)]] on a deduced return type (auto / decltype(auto)).

Minimal reproducer (clang-cl --target=aarch64-pc-windows-msvc -std:c++latest -c repro.cpp):

```c++
template
constexpr auto [[clang::annotate_type("foo")]] func(const char(&p)[N])
{
return N;
}

void caller() {
func("hello");
}
```

Clang version 21.1.8. This code compiles on Clang 20.

Start by reproducing the crash with clang-cl --target=aarch64-pc-windows-msvc -std:c++latest -c repro.cpp using the minimal template in the issue, then trace the failure in the MSVC ABI mangler. Done means this reproducer compiles without a crash and the behavior is covered by an appropriate regression test.

Written by the indexing model from the issue text.