Temporaries generated by LocalRewriter for collection expression spans should be marked as long-lived

[cston]

The temporaries generated by LocalRewriter for collection expression spans are created at the outermost scope of the method and span multiple statements or sequence points, and therefore should be considered long-lived locals.

Currently however, the temporaries are created with SynthesizedLocalKind.LoweringTemp, for which SynthesizedLocalExtensions.IsLongLived(kind) returns false.

The fact that the temp locals are not considered long-lived may cause issues for the following:

• EnC which matches locals across method versions
• SpillSequenceSpiller which rewrites locals that are not long-lived (although currently ref struct instances cannot be used across await)

Based on comment and discussion with @AlekseyTs.

[cston]

cc @tmat