Fixup path fragments upon MBE transcription
Fixes#14367
There are roughly two types of paths: paths in expression context, where a separator `::` between an identifier and its following generic argument list is mandatory, and paths in type context, where `::` can be omitted.
Unlike rustc, we need to transform the parsed fragments back into tokens during transcription. When the matched path fragment is a type-context path and is transcribed as an expression-context path, verbatim transcription would cause a syntax error.
This PR fixes up path fragments by inserting `::` to make sure they are syntactically correct in all contexts. Note that this works because expression-context paths are a strict superset of type-context paths.
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