Optimize open-file-only path of ParseProjectJob::queueFilesToParse()

Let the more efficient loop below queue each set element instead of
searching for each open document in the set gradually emptying it.
1 job for !177 with fix-parseprojectjob-bugs in 60 minutes and 39 seconds (queued for 100 minutes and 37 seconds)
detached
Status Job ID Name Coverage
  Build
failed #71419
linux kf5-qt5 qt5.12

01:00:39

45.0%
 
Name Stage Failure
failed
linux kf5-qt5 qt5.12 Build The script exceeded the maximum execution time set for the job
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Uploading artifacts for successful job
Uploading artifacts...
build: found 16392 matching files and directories

WARNING: Uploading artifacts as "archive" to coordinator... failed id=71419 responseStatus=502 Bad Gateway status=502 token=EyFAoJUg
WARNING: Retrying... context=artifacts-uploader error=invalid argument
WARNING: Uploading artifacts as "archive" to coordinator... failed id=71419 responseStatus=502 Bad Gateway status=502 token=EyFAoJUg
WARNING: Retrying... context=artifacts-uploader error=invalid argument
ERROR: Job failed: execution took longer than 1h0m0s seconds