Only warn on duplicate members within a single descriptors file

[jtschuster]

In the trimmer we rely on the order of marking to determine if a descriptors file has a duplicate member. As we transition to the dependency analysis framework, the marking order changes, and this exposed scenarios where a descriptors file is found after some items have already been marked. This caused unnecessary IL2025 warnings (Duplicate preserve in descriptor file) to be reported. Instead of using the global marking state, we should keep a per-descriptor-file set of which members are preserved and only report duplicates within that set.

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[MichalStrehovsky]

This looks good, but I always wondered why we need to warn.

Could this warn in legitimate situations, like embedded descriptors in two different XML files preserving the same thing, or even within the same descriptor if the preservations are under different feature flags and we happen to enable both feature flags at the same time, leading to both being used?

[jtschuster]

This shouldn't warn on the same member being marked in different embedded XML files, a new object is created for each descriptor file with a new _preservedMembers set. However, I think it would warn if the member is listed twice for different feature flags and both are enabled.

I'm fine removing the warnings, I don't see a lot of value in them.

[sbomer]

Looks like it was introduced as a convenience to help clean up some descriptor files in dotnet/linker#293. I'm fine with removing them now that we've added feature switches. Another option is to make it an info message instead.

[sbomer]

LGTM, thank you!