`getOrBuildFir` does not record `FirCheckedSafeCallSubject`
expression, and it would be impossible to get them anyway
since there are no PSI corresponding to "not null receiver"
in expressions like `foo?.bar` - there is only `foo`,
which is considered nullable by FIR
`FirCheckedSafeCallSubject` wrapper has non-null type during
resolve, and it becomes very important in
`KtFirCompletionCandidateChecker`. If we loose the wrapper,
then it would be impossible to call extensions on nullable
types using `?.` syntax
^KTIJ-21021 Fixed
Roman Golyshev
42010282d5