Allow multiple typedefs for same type (#950)

* Allow multiple typedefs for same type

Author: kant2002

Cesium.CodeGen.Tests/CodeGenTypeTests.cs

@@ -466,4 +466,13 @@ void foo(foo* p1, foo* p2) {
     *p1 = *p2;
 }
 """);
+
+    [Fact]
+    public Task TypeDefTwice() => DoTest(@"typedef struct Token Token;
+typedef struct Token Token;
+
+struct Token {
+    int x;
+};
+");
 }

Cesium.CodeGen.Tests/verified/CodeGenTypeTests.TypeDefTwice.verified.txt

@@ -0,0 +1,6 @@
+Module: Primary
+  Type: <Module>
+
+  Type: Token
+  Fields:
+    System.Int32 Token::x

Cesium.CodeGen/Contexts/TranslationUnitContext.cs

@@ -132,7 +132,16 @@ internal void GenerateType(string name, StructType type)
     internal void AddTypeDefinition(string name, IType type)
     {
         if (_types.ContainsKey(name))
+        {
+            if (type is StructType newStructType && newStructType.Members.Count == 0)
+            {
+                if (_types[name] is StructType existingStructType && existingStructType.Members.Count == 0 && existingStructType.Identifier == newStructType.Identifier)
+                {
+                    return;
+                }
+            }
             throw new CompilationException($"Type definition {name} was already defined.");
+        }
 
         _types.Add(name, type);
     }