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/irtypefunction.h

@@ -23,18 +23,17 @@ class IrTypeFunction : public IrType
 {
 public:
     ///
-    IrTypeFunction(Type* dt);
+    static IrTypeFunction* get(Type* dt);
 
     ///
     IrTypeFunction* isFunction()  { return this; }
 
-    ///
-    llvm::Type* buildType();
-
     IrFuncTy* fty() { return irfty; }
 
 protected:
-    llvm::Type* func2llvm(Type* dt);
+    ///
+    IrTypeFunction(Type* dt, llvm::Type* lt);
+
     ///
     IrFuncTy* irfty;
 };
@@ -44,15 +43,14 @@ class IrTypeDelegate : public IrType
 {
 public:
     ///
-    IrTypeDelegate(Type* dt);
+    static IrTypeDelegate* get(Type* dt);
 
     ///
     IrTypeDelegate* isDelegate()    { return this; }
 
-    ///
-    llvm::Type* buildType();
 protected:
-    llvm::Type* delegate2llvm(Type* dt);
+    ///
+    IrTypeDelegate(Type* dt, llvm::Type* lt);
 };
 
 #endif