Fix provablyDisjoint handling enum constants with mixins #33019

Job	Run time
test_non_bootstrapped	0s
test	57m 8s
test_windows_fast	39m 5s
test_scala2_library_tasty	0s
MiMa	12m 1s
test_windows_full	0s
community_build_a	50m 56s
community_build_b	1h 21m 38s
community_build_c	59m 43s
test_sbt	18m 26s
build-msi-package	0s
test_java8	0s
build-sdk-package / build	7m 47s
test-msi-package	0s
build-chocolatey-package / build	20s
test-chocolatey-package	0s
publish_nightly	0s
publish_release	0s
nightly_documentation	0s
open_issue_on_failure	0s
	5h 27m 4s

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