Add `/warnVacuousProofs` option #1956

Job	Run time
Boogie CI (Release, batch_mode=True)	4m 40s
Boogie CI (Debug, batch_mode=True)	4m 26s
Boogie CI (Debug, batch_mode=False)	4m 31s
Boogie CI (Release, batch_mode=False)	4m 40s
	18m 17s

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