UnresolvedMessages.GetMessageAsync is allocating more heavily than necessary

[ToddGrun]

NuGet Product Used

Other/NA

Product Version

VS 17.14

Worked before?

No response

Impact

It bothers me. A fix would be nice

Repro Steps & Context

CSharpEditingTests.ScrollingAndTyping speedometer test shows 0.6% of all VS allocations from VS start until opening the file under UnresolvedMessages.GetMessageAsync.

The primary issue here is the use of a SortedSet as it's ctor allocates a fair bit, and each node in the set requires an allocation for the wrapping node. Instead, the code could be modified to not require the collection to be sorted without any performance impact.

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