Add `/warnVacuousProofs` option #1961

Job	Run time
Boogie CI (Release, batch_mode=True)	4m 19s
Boogie CI (Debug, batch_mode=True)	4m 24s
Boogie CI (Release, batch_mode=False)	4m 35s
Boogie CI (Debug, batch_mode=False)	4m 35s
	17m 53s

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