Currently only ConstantArrayType is considered for flexible array
members (FAMs) in getStaticSize().
However, IncompleteArrayType also shows up in practice as FAMs.
This patch will ignore the IncompleteArrayType and return Unknown
for that case as well. This way it will be at least consistent with
the current behavior until we start modeling them accurately.
I'm expecting that this will resolve a bunch of false-positives internally,
caused by the ArrayBoundV2.