Trim analyzer should warn about assignment to get-only property

[sbomer]

Opening this to track the follow-up items discussed in dotnet/linker#2728 - see that PR for full context.

The analyzer doesn't produce any warnings when assigning to a get-only property, because it doesn't know about the compiler-generated backing field. To fix this we could either:

• continue using the same warning message, but with the property name instead of the compiler-generated field name that the linker see, or
• reword the message to avoid mentioning "field" specifically, so it can be used for both the linker and analyzer.

[vitek-karas]

The analyzer should basically talk about writing to the property. If it means adapting the message to make it reusable, let's go with that.

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Issue Details

---

Opening this to track the follow-up items discussed in dotnet/linker#2728 - see that PR for full context.

The analyzer doesn't produce any warnings when assigning to a get-only property, because it doesn't know about the compiler-generated backing field. To fix this we could either:

• continue using the same warning message, but with the property name instead of the compiler-generated field name that the linker see, or
• reword the message to avoid mentioning "field" specifically, so it can be used for both the linker and analyzer.

Author:	sbomer
Assignees:	-
Labels:	untriaged, area-Tools-ILLink, needs-area-label
Milestone:	-