Go: minor improvements

[masatake]

(Bugs should not be listed here. Please, open an new issue.)

• support generics Go: support generics #4397
• capture parameters
• capture local variables Go: Declarations inside functions are not tagged #3454
• support union Go: support union #4398

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Done:

• capture receivers
• capture variables for return (5a8e67d)
• support end: field