Don't warn if we can't find the backing field in trim tools

[jtschuster]

Fixes #119780

We added a warning when we cannot find the backing field of an auto property in #119329 which caused some issues in illink->ilc scenarios when an auto property was stubbed out. Instead, we should just not propagate the DAM annotation.

[Copilot]

Pull Request Overview

This PR addresses a warning issue in trim tools where IL2042 warnings were incorrectly generated when backing fields of auto properties couldn't be found, particularly in illink->ilc scenarios when auto properties were stubbed out. Instead of warning, the tools now simply skip propagating the DAM annotation when the backing field cannot be located.

Key Changes

• Remove IL2042 warning generation when backing fields of auto properties cannot be found
• Simplify backing field detection logic to silently handle missing fields
• Update Roslyn analyzer to only check for auto properties when field has an associated property

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
PropertyDataFlow.cs	Remove expected IL2042 warnings from test cases since these warnings are no longer generated
FlowAnnotations.cs (illink)	Simplify backing field logic to skip warning when field not found and only propagate annotation when valid backing field exists
DiagnosticId.cs	Mark IL2042 diagnostic ID as unused by prefixing with "unused_"
FlowAnnotations.cs (analyzer)	Optimize property checking by combining auto property check with property symbol check
FlowAnnotations.cs (ilcompiler)	Apply same backing field logic simplification as illink version

[sbomer]

LGTM, thank you!

[agocke]

Thanks!

[agocke]

/ba-g unknown failures were crashes in the infra