Dotty

Backport "Drop phase.isTyper use in isLegalPrefix/asf" to 3.3 LTS #250

Job	Run time
test	27m 15s
test_windows_fast	22m 34s
community_build_a	1h 2m 38s
community_build_b	1h 36m 11s
community_build_c	1h 7m 21s
MiMa	3m 54s
test_java8	0s
test_non_bootstrapped	0s
test_sbt	16m 18s
test_windows_full	0s
build-sdk-package / build	4m 28s
build-chocolatey-package / build	17s
test-chocolatey-package	0s
publish_nightly	0s
publish_release	0s
nightly_documentation	0s
open_issue_on_failure	0s
	5h 0m 56s

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