Essentially, having a default member initializer for a constant member
does not necessarily imply the member will have the given default value.
Remove part of a2e053638b ([analyzer] Treat more const variables and
fields as known contants., 2018-05-04).
Fix#47878
Reviewed By: r.stahl, steakhal
Differential Revision: https://reviews.llvm.org/D124621
Marco Antognini