As discussed in the review of D59217, this member is unnecessary since
always the first thing we do is convert it to a CompilerType.
This opens up possibilities for further cleanups (e.g. the whole
TypePair class now loses purpose, since we can just pass around
CompilerType everywhere), but I did not want to do that yet, because I
am not sure if this will not introduce breakages in some of the
platforms/configurations that I am not testing on.