Prefer C++-style casts.

This is based on Item 2 of "More Effective C++". In general, the C++ cast operators are more expressive and easy to find,
e.g. by grep. Using const_cast also shuts up some compiler warnings.

ir/irfunction.cpp

@@ -129,7 +129,7 @@ IrFunction::IrFunction(FuncDeclaration* fd)
 
     Type* t = fd->type->toBasetype();
     assert(t->ty == Tfunction);
-    type = (TypeFunction*)t;
+    type = static_cast<TypeFunction*>(t);
     func = NULL;
     allocapoint = NULL;