Fix issue with extension members and unfound doc comment references

[srawlins]

Fixes #3808

The main issue is that #3805 changed how extension accessors and "fields" (the synthetic ones introduced by accessors) are instantiated. This tidies up a few places related to that. The main thrust of this is in container_member.dart, where we prefer the canonical element of a field's getter/setter, rather than the canonical element of the field, which can introduce a weird side effect of stranded accessors without an "enclosingCombo" of the field.

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[srawlins]

CC @dart-lang/analyzer-team