[analyzer] Ignore IncompleteArrayTypes in getStaticSize() for FAMs

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`.

Reviewed By: ASDenysPetrov

Differential Revision: https://reviews.llvm.org/D105184