Add diagnostics for type alias to a wildcard, closes #357

In the current implementation of cppfront (f83ca9) the following code:
```cpp
alias5: type == _;
```
Generates succesfuly:
```cpp
using alias5 = auto;
```
Which is invalid cpp1 code.

After this change the alias to wildcard will generate the following error:
```
a 'type ==' alias declaration must be followed by a type name and not a wildcard _
```

All regression tests pass. Closes #357

Author: filipsajdak

source/parse.h

@@ -5484,6 +5484,13 @@ class parser
                 );
                 return {};
             }
+            if (t->is_wildcard()) {
+                errors.emplace_back(
+                    curr().position(),
+                    "a 'type ==' alias declaration must be followed by a type name and not a wildcard _"
+                );
+                return {};
+            }
             a->initializer = std::move(t);
         }