Remove TIntermediate::addConstantUnion This includes asserts in TConstantUnion to reveal incorrect usage of union - reading a different field of an union that has last been set is undefined behavior in C++. Existing issues with accessing incorrect fields of constant unions are fixed. BUG=angleproject:1490 TEST=angle_unittests Change-Id: Idd6b7a871d73e2928f117a9348c92043612fab82