Update handling of varargs in unannotated / unmarked code

[msridhar]

Follow-up to #1025. If the varargs argument is in unannotated code and restrictive annotations are disabled, callers from marked code should be able to pass @Nullable either for individual varargs arguments or a @Nullable varargs array.

With restrictive annotations enabled, hopefully this table should cover the behaviors when we see explicit @NonNull annotations (legacy mode behavior to be confirmed):

Annotation Type	Annotation Position	Mode	@NonNull varargs array	@NonNull individual args
declaration	-	standard	❌	✅
declaration	-	legacy	✅	✅
declaration	-	JSpecify	❌	✅
type use	before ...	standard	✅	❌
type use	before ...	legacy	✅	✅
type use	before ...	JSpecify	✅	❌
type use	elements	standard	❌	✅
type use	elements	legacy	✅	✅
type use	elements	JSpecify	❌	✅
type use	both	standard	✅	✅
type use	both	legacy	✅	✅
type use	both	JSpecify	✅	✅
both	before ...	standard	✅	❌
both	before ...	legacy	✅	✅
both	before ...	JSpecify	✅	❌
both	elements	standard	❌	✅
both	elements	legacy	✅	✅
both	elements	JSpecify	❌	✅
both	both	standard	✅	✅
both	both	legacy	✅	✅
both	both	JSpecify	✅	✅

Note that restrictive annotations are read from @NullUnmarked code by default, so calls into @NullUnmarked code should behave as per the table even if the restrictive annotations flag is not passed.