Remove temporary allocations when calling cmGeneratorTarget::GetName.

This happens quite often from within comparisons such as in
NamedGeneratorTargetFinder or FindGeneratorTargetImpl. It is the
top hotspot of both, number of allocations as well as number of
temporary allocations - the majority of calls lead to temporary
allocations.

In raw numbers, this patch removes ~1E6 temporary allocations of
1.5E6 temporary allocations in total when running the cmake daemon
on the KDevelop build dir. That is 2/3 of the total.

This hotspot was found with heaptrack.