Update scala-library, scala-reflect to 2.12.20 (#225) #596

Job	Run time
Build and Test (8, 2.12)	2m 54s
Build and Test (8, 2.13)	2m 48s
Build and Test (8, 3.3)	3m 6s
Build and Test (11, 2.12, -J-XX:+UnlockExperimentalVMOptions -J-XX:+UseJVMCICompiler)	3m 24s
Build and Test (11, 2.13, -J-XX:+UnlockExperimentalVMOptions -J-XX:+UseJVMCICompiler)	3m 9s
Build and Test (11, 3.3, -J-XX:+UnlockExperimentalVMOptions -J-XX:+UseJVMCICompiler)	3m 10s
Build and Test (17, 2.12)	3m 6s
Build and Test (17, 2.13)	3m 7s
Build and Test (17, 3.3)	3m 8s
Build and Test (21, 2.12)	2m 59s
Build and Test (21, 2.13)	3m 7s
Build and Test (21, 3.3)	2m 47s
	36m 45s

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