[libcxxabi][ItaniumDemangle] Demangle explicitly named object parameters (#72881)

The mangling for an explicitly named object was introduced in
https://reviews.llvm.org/D140828

See following discussion for why a new mangling had to be introduced:
https://github.com/itanium-cxx-abi/cxx-abi/issues/148

Since clang started emitting names with the new mangling, this patch
implements support for demangling such names.

The approach this patch takes is to add a new `ExplicitObjectParameter`
node that will print the first parameter of a function declaration with
a `this ` prefix, to reflect what was spelled out in source.

Example:
```
void MyClass::func(this MyClass const& self); // _ZNH7MyClass4funcERKS_
```
With this patch, the above demangles to:
```
_ZNH7MyClass4funcERKS_ -> MyClass::func(this MyClass const&)
```

Note that `func` is not marked as `const &`, since the
function-qualifiers are now encoded as part of the explicit `this`. C++
doesn't allow specifying the function-qualifiers in the presence of an
explicit object parameter, so this demangling is consistent with the
source spelling.