Issue 20339: isPOD returns true if sizeof is accessed inside struct declaration
Issue 24292: Struct with destructor wrongly returned in register
Function StructDeclaration.isPOD can be called before the semantic run
for the struct is finished. It then uses incomplete information about
destructors, postblits and copy constructors. The result of isPOD is
cached, so later calls will also return the wrong result.
This commit changes isPOD, so it uses variables, which are already
filled before the semantic run.
