Dotty

Allow discarding "Discarded non-Unit" warnings with `: Unit` #33152

Job	Run time
test_non_bootstrapped	0s
test	34m 14s
community_build_a	34m 50s
community_build_b	33m 9s
MiMa	12m 40s
test_scala2_library_tasty	0s
test_windows_fast	17m 46s
test_windows_full	0s
community_build_c	19m 8s
build-msi-package	0s
test_sbt	33s
test_java8	0s
build-sdk-package / build	8m 10s
test-msi-package	0s
build-chocolatey-package / build	18s
test-chocolatey-package	0s
publish_nightly	1s
publish_release	1s
nightly_documentation	1s
open_issue_on_failure	1s
	2h 40m 52s

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