[clang] [clang][ItaniumMangle] Fix mangling of lambdas in default member initializers of local classes (PR #206740)

[Erich Keane via cfe-commits]

erichkeane wrote:

> @erichkeane Thank you for the feedback! I'll add a release note.
> 
> > However, this ALSO needs to have an entry in the -fclang-abi list, this is obviously quite breaking of a change ABI wise.
> 
> I've thought about this, but I'm not yet convinced that a new `-fclang-abi-compat` entry is necessary. The change only affects function-local classes, and while such types can technically escape their function scope, they seems cannot escape the translation unit in a way that would create a ABI compatibility issue. Am I missing something? As a point of comparison, a similar mangling fix in [PR #195340](https://github.com/llvm/llvm-project/pull/195340) did not introduce a new ABI compatibility entry. That said, I'm happy to add one, just want to make sure it is actually necessary.
> 
> > Does our lambda numbering still work correctly? it isn't clear from your tests.
> 
> I'm not intending to change lambda numbering, but I checked it for sure. This example:
> 
> ```
>   void foo() {
>     {
>       auto f = [] { std::cout << 1 << '\n'; };
>       f();
>     }                                                                                                                                                                                                            
>   
>     {
>       auto f = [] { std::cout << 2 << '\n'; };
>       f();
>     }
>   }
> ```
> 
> produces a bit unusual mangling, but the numbering itself seems correct and isn't affected by the patch:
> 
> ```
> _ZZ3foovENK3$_0clEv
> _ZZ3foovENK3$_1clEv
> ```
> 
> But I'd expect smth like this anyway
> 
> ```
> _ZZ3foovENKUlvE_clEv
> _ZZ3foovENKUlvE0_clEv
> ```

Hmm... we perhaps SHOULD have on that other patch...  

```
Am I missing something?
```
Perhaps? Every time I personally determine "we probably don't need one because there is no WAY this can escape the TU!" I've been wrong :D  

https://github.com/llvm/llvm-project/pull/206740