Optimize Type::isStructureOrClassType() by reusing RT->getDecl().

RecordType->getDecl() which maps to TagType::getDecl() is not a simple
accessor but a loop on redecls in getInterestingTagDecl.

isStructureOrClassType() was calling getDecl() three times performing
three times the work actually required. It is optimized by calling
RT->getDecl() once and reusing the result three times.