Use existing TypeParameterReference instances instead of creating new ones.

ICSharpCode.Decompiler/Metadata/Dom.cs

@@ -882,7 +882,7 @@ namespace ICSharpCode.Decompiler.Metadata
 
 		public FullTypeName GetPrimitiveType(PrimitiveTypeCode typeCode)
 		{
-			return new FullTypeName($"System.{typeCode}");
+			return new TopLevelTypeName("System", typeCode.ToString());
 		}
 
 		public FullTypeName GetSZArrayType(FullTypeName elementType)

ICSharpCode.Decompiler/TypeSystem/DecompilerTypeSystem.cs

@@ -506,7 +506,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 				return this;
 			} else {
 				return new SpecializingDecompilerTypeSystem(this, substitution);
-	}
+			}
 		}
 	}
 }

ICSharpCode.Decompiler/TypeSystem/Implementation/TypeSpecification.cs

@@ -237,12 +237,12 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 
 		public ITypeReference GetGenericMethodParameter(Unit genericContext, int index)
 		{
-			return new TypeParameterReference(SymbolKind.Method, index);
+			return TypeParameterReference.Create(SymbolKind.Method, index);
 		}
 
 		public ITypeReference GetGenericTypeParameter(Unit genericContext, int index)
 		{
-			return new TypeParameterReference(SymbolKind.TypeDefinition, index);
+			return TypeParameterReference.Create(SymbolKind.TypeDefinition, index);
 		}
 
 		public ITypeReference GetModifiedType(ITypeReference modifier, ITypeReference unmodifiedType, bool isRequired)