Since there is no reliable way to do this, I suggest planning a script that will clear it. For example, to clear old subfolders that were not accessible after 30 days:
INSTFLD="/Library/Caches/com.apple.dt.instruments"
find $INSTFLD -type d -atime +30 -mindepth 1 -maxdepth 1 -exec rm -rf {} \;
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