Ensure System.Text.Json SG diagnostics don't use locations from external assemblies

[eiriktsarpalis]

          We can consider making a type annotated with `[OptionsValidator]` to be a fallback in that case (worst case scenario is when a validator is applied on a type residing in another assembly).

For JSON source-gen, a class annotated with [JsonSerializable] can be considered as a fallback location.

Originally posted by @xakep139 in #88396 (comment)

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Issue Details

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          We can consider making a type annotated with `[OptionsValidator]` to be a fallback in that case (worst case scenario is when a validator is applied on a type residing in another assembly).

For JSON source-gen, a class annotated with [JsonSerializable] can be considered as a fallback location.

Originally posted by @xakep139 in #88396 (comment)

Author:	eiriktsarpalis
Assignees:	-
Labels:	area-System.Text.Json, untriaged
Milestone:	-