[Analyzer] Fix assumptions about const field with member-initializer

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 a2e05363 ([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