ProhibitAssigningSingleElementsToVarargsInNamedForm is enabled from 1.3
so there is no need to make FIR accepting single element passed through
named arguments.
In addition, we may want to report only
ASSIGNING_SINGLE_ELEMENT_TO_VARARG_IN_NAMED_FORM_FUNCTION and not arg
type mismatch. But FE1.0 is reporting both so I am following it. If we
want to remove the redundant report, we will probably want to remove
FirNamedVarargChecker and report during resolution as a cone diagnostic.
Tianyu Geng