[clang-tidy] Fix handling of UseAssignment option in cppcoreguidelines-prefer-member-initializer

From now on check will use value from cppcoreguidelines-prefer-member-initializer
and fallback to modernize-use-default-member-init.UseAssignment if not specified.

Fixes: #55616.

Reviewed By: carlosgalvezp

Differential Revision: https://reviews.llvm.org/D147929