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