[clang] [CodeGen] Disable exact dynamic_cast for duplicable vtables (PR #205929)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> ` !AssumeUniqueVTable` disables the optimization unconditionally, including for classes with key functions. A key-function class vtable has one strong definition, so it does have a unique address.
> 
> Also, it's not clear to me what ` !AssumeUniqueVTable` means. `AssumeUniqueVTable` means "assume a class has only one vtable", and that's used by the dynamic_cast optimization to determine whether it's safe to perform the optimization. Its negation only tells the optimizer not to rely on vtable uniqueness. I don't think it also means the platform allows emitting vtables in different linkage units and therefore vtables can be annotated with `unnamed_addr` (see #205930).

!AssumeUniqueVTable means precisely that: the same vtable may have multiple addresses.

That said I do see that the constraint here is different.

My suggestion would be to change `AssumeUniqueVTable` into an enum type rather than a boolean flag. That means there's a single place describing the behavior, the options should cover

1. unnamed_addr is valid on all vtables
2. unnamed_addr is valid on some vtables 
3. unnamed_addr is never valid on tables -- which means vtables can be assumed to be unique

e.g.

```cpp
enum class BetterNameForVTableUniqueness {
  AssumedUnique, // unnamed_addr is never valid
  ConditionallyUnique, // unnamed_addr is sometimes allowed
  NeverUnique // unnamed_addr is always allowed
}

bool betterNameForCanUseUnnamedAddressForVTable(CXXRecordDecl *Class, BetterNameForVTableUniqueness Uniqueness) {
  switch (Uniqueness) {
  case BetterNameForVTableUniqueness::AssumedUnique:
    return false;
  case BetterNameForVTableUniqueness::ConditionallyUnique:
   return Class->hasRequiredPropertyToHaveUnnamedAddr()
  default:
   return true;
  }
}

bool betterNameForVTableCanBeAssumedUnique(CXXRecordDecl *Class, BetterNameForVTableUniqueness Uniqueness) {
  switch (Uniqueness) {
  case BetterNameForVTableUniqueness::AssumedUnique:
    return true;
  case BetterNameForVTableUniqueness::ConditionallyUnique:
   return !Class->hasRequiredPropertyToHaveUnnamedAddr(); // note !
  default:
   return false;
  }
}
```

Or something to that effect

https://github.com/llvm/llvm-project/pull/205929