[cling] In failed lookup, unload only decls produced, not existing fwd decl:

Before, pre-existing fwd decls of specializations got unloaded.

OTOH, any decl produced during (failed) template instantiation must
also be unloaded; see #6331. Those are easiest identified by unloading
the whole (failed) transaction.

```
error: no member named 'value' in 'std::__and_<std::is_copy_assignable<std::__cxx11::basic_string<char> >, std::is_copy_assignable<Inner<int> > >'
                       is_copy_assignable<_T2>>::value,
                       ~~~~~~~~~~~~~~~~~~~~~~~~~~^
```

which is due to the first decl being invalid (as `Inner<int>` does not have a deinition and a `static_assert` /usr/lib/gcc/x86_64-redhat-linux/10/../../../../include/c++/10/type_traits:1093 being triggered), then not unloaded, and then picked up again where we *do* have the definition for `Inner<int>`.

Add test for templt spec lookup vs unloading.