Unify internal tracking of warning context

Opening this to track the future improvements discussed in https://github.com/mono/linker/issues/2163:

We currently have a few ways to track the warning context/source (or the reason that we marked something): DependencyInfo, MarkScopeStack, and ReflectionPatternContext. When we go to mark a method, for example, we use this information to determine:
- which warnings to generate
- which warning origin to use, and
- which RequiresUnreferencedCodeAttributes to look for that may silence the warning

We should consider replacing these with one solution that covers all of those use cases. See the discussion in https://github.com/mono/linker/issues/2163 for more context.