Handle ILLink origins before first sequence point

[sbomer]

Fixes#129562.

MessageOrigin.ToString() assumed that every diagnostic IL offset has at least one PDB sequence point at or before that offset. Rewritten/generated IL can put a warning origin before the first sequence point, which made Last() throw while ILLink flushed cached warnings.

This changes the lookup to tolerate that shape by falling back to the first available sequence point, and adds a focused regression test that round-trips a Cecil-generated assembly/PDB to reproduce the real offset behavior.

This content was created with assistance from AI.

[dotnet-policy-service]

Tagging subscribers to this area: @agocke, @dotnet/illink
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

This PR updates ILLink’s diagnostic origin formatting to handle method IL offsets that occur before the first available PDB sequence point, preventing MessageOrigin.ToString() from throwing when generating MSBuild-formatted messages. It also adds a regression test that constructs and round-trips a Cecil-generated assembly + Portable PDB to reproduce the offset shape.

Changes:

• Make MessageOrigin.ToString() tolerate “origin before first sequence point” by falling back to the first available sequence point instead of throwing.
• Add a targeted xUnit regression test that emits a Portable PDB where the only sequence point is after the first IL instruction, then verifies ToMSBuildString() output.

Show a summary per file

File	Description
src/tools/illink/src/linker/Linker/MessageOrigin.cs	Adjusts sequence point selection logic to avoid Last() throwing when no sequence point exists at/before the IL offset.
src/tools/illink/test/Mono.Linker.Tests/Tests/MessageContainerTests.cs	Adds a Cecil-based regression test covering an origin IL offset occurring before the first sequence point.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 1