[ruby/prism] Fix parser translator with trailing backslash in `%W` /`… #629

Job	Run time
omnibus compilations, trigger	4s
omnibus compilations, #5	26m 49s
omnibus compilations, #1	26m 56s
omnibus compilations, #2	34m 7s
omnibus compilations, #7	32m 42s
omnibus compilations, #6	27m 17s
omnibus compilations, #3	30m 58s
omnibus compilations, #9	26m 30s
omnibus compilations, #8	27m 11s
omnibus compilations, #4	25m 10s
omnibus compilations, #11	27m 35s
omnibus compilations, #10	26m 13s
omnibus compilations, #12	24m 5s
omnibus compilations, result	5s
	5h 35m 42s

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