feat: Add support for C++ style forward declares.

In cimple, `struct X;` will now mean the same as `typedef struct X X;`,
which is (roughly) the same behaviour as in C++. This is only to support
having plain `struct X;` declarations where we put our comments vs.
having the typedefs surrounded by ifdefs for C99 reasons (benign typedef
redefinitions are a C11 feature).

src/Language/Cimple/Parser.y

@@ -666,6 +666,7 @@ TypedefDecl :: { NonTerm }
 TypedefDecl
 :	typedef QualType ID_SUE_TYPE ';'			{ Fix $ Typedef $2 $3 }
 |	typedef FunctionPrototype(ID_FUNC_TYPE) ';'		{ Fix $ TypedefFunction $2 }
+|	struct ID_SUE_TYPE ';'					{ Fix $ Typedef (Fix (TyStruct $2)) $2 }
 
 QualType :: { NonTerm }
 QualType