[clang] Revert "[DebugInfo] Ignore undefined constexpr constructors in constructor homing." (PR #221566)

[Avi Kivity via cfe-commits]

avikivity wrote:

A self-contained, linkable reproducer, in the shape of `absl::container_internal::map_slot_type<K, V>`: the constructor definitions are present (as libstdc++'s `std::pair` constructors are), but are never instantiated for `mypair<const K, V>`, because construction goes through `mutable_value` and only reads go through `value`.

```c++
template <class A, class B>
struct mypair {
    A first;
    B second;
    constexpr mypair() : first(), second() {}
    constexpr mypair(const A& a, const B& b) : first(a), second(b) {}
};

template <class K, class V>
union map_slot_type {
    map_slot_type() {}
    ~map_slot_type() {}
    using value_type = mypair<const K, V>;          // read, never constructed
    using mutable_value_type = mypair<K, V>;        // constructed
    value_type value;
    mutable_value_type mutable_value;
    K key;
};

map_slot_type<unsigned long, void*> slot;
int marker;

int main() {
    // Construction goes through the mutable pair, as absl's map_slot_policy does.
    slot.mutable_value = mypair<unsigned long, void*>(42, &marker);
    // Reads go through the const-key pair, whose type is now unavailable.
    return (slot.value.first == 42 && slot.value.second == &marker) ? 0 : 1;
}
```

```
$ clang++ -O2 -g ctor-homing-linkable.cc -o prog && ./prog; echo $?
0
$ llvm-dwarfdump --debug-info prog | grep -A4 'DW_AT_name.*"mypair<const'
$ gdb -q -batch -ex 'set language c++' -ex 'ptype mypair<const unsigned long, void*>' prog
```

clang 22.1.8:

```
DW_AT_name	("mypair<const unsigned long, void *>")
DW_AT_byte_size	(0x10)

type = struct mypair<unsigned long const, void*> [with A = const unsigned long, B = void *] {
    const A first;
    B second;
    mypair(void);
    mypair(const A &, void * const&);
}
```

clang 24.0.0git (03de3d1299bf, i.e. with 6ba0802b406e):

```
DW_AT_name	("mypair<const unsigned long, void *>")
DW_AT_declaration	(true)

No symbol "mypair<const unsigned long, void*>" in current context.
```

Both link and run, exit status 0.

Dropping `constexpr` from both constructors, everything else unchanged:

```
clang 22.1.8     exit=0  const-key pair: DW_AT_declaration
clang 24.0.0git  exit=0  const-key pair: DW_AT_declaration
```


https://github.com/llvm/llvm-project/pull/221566