[llvm] [llvm] [Demangle] Fix MSVC demangling for placeholder return types (PR #106178)

[Max Winkler via llvm-commits]

MaxEW707 wrote:

> Can't you just add these to `PrimitiveKind` and add the match cases to the end of `demanglePrimitiveType`? It already tries to parse `_*`:
> 
> https://github.com/llvm/llvm-project/blob/7c188abb4fe28d9784b12125e6924df2ce7de078/llvm/lib/Demangle/MicrosoftDemangle.cpp#L2016-L2040
> 
> I don't see why you need to create and entirely new node type for something so simple.

Agreed. I assumed that wouldn't fly since pedantically `auto` and `decltype(auto)` aren't considered primitive types but I probably should have went with the easiest solution first.

I'll change the code to just add the two new cases to `demanglePrimitiveType`.

https://github.com/llvm/llvm-project/pull/106178