Add diagnostics for type alias to a wildcard, closes #357 (#360)

In the current implementation of cppfront (f83ca9) the following code:
```cpp
alias5: type == _;
alias6: type == auto;
```
Generates succesfuly:
```cpp
using alias5 = auto;
using alias6 = auto;
```
Which is invalid cpp1 code.

After this change the alias to wildcard will generate the following error:
```
error: a 'type ==' alias declaration must be followed by a type name (not a wildcard _ nor auto)
```

All regression tests pass. Closes #357

source/parse.h

@@ -5628,6 +5628,16 @@ class parser
                 );
                 return {};
             }
+            if (
+                t->is_wildcard() 
+                || ( t->get_token() && t->get_token()->to_string(true) == "auto" )
+            ) {
+                errors.emplace_back(
+                    curr().position(),
+                    "a 'type ==' alias declaration must be followed by a type name (not a wildcard _ nor auto)"
+                );
+                return {};
+            }
             a->initializer = std::move(t);
         }