Refactored IrType construction to use static get() method.

This also allows us to enable the assert in IrType::IrType.

Unfortunately, this is mostly a "peace of mind" commit, there
doesn't seem to have been a bug actually caused by the transitory
duplicate IrTypePointer/IrTypeStruct instances.

The remaining xyz2llvm static methods are not exactly pretty,
they should probably just be folded into get.

ir/irtypeclass.h

@@ -22,14 +22,11 @@ class IrTypeClass : public IrTypeAggr
 {
 public:
     ///
-    IrTypeClass(ClassDeclaration* cd);
+    static IrTypeClass* get(ClassDeclaration* cd);
 
     ///
     virtual IrTypeClass* isClass()      { return this; }
 
-    ///
-    llvm::Type* buildType();
-
     ///
     llvm::Type* getLLType();
 
@@ -53,6 +50,9 @@ public:
     unsigned getNumInterfaceVtbls()     { return num_interface_vtbls; }
 
 protected:
+    ///
+    IrTypeClass(ClassDeclaration* cd);
+
     ///
     ClassDeclaration* cd;
     ///