FIR: don't add T <: Unit constraints that will only remove errors

^KT-55693 Fixed

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/inference/FirCallCompleter.kt

@@ -184,7 +184,7 @@ class FirCallCompleter(
                     )
                 }
             }
-            !expectedType.isUnitOrFlexibleUnit || !mayBeCoercionToUnitApplied -> {
+            !expectedType.isUnitOrFlexibleUnit || (!mayBeCoercionToUnitApplied && !expectedTypeMismatchIsReportedInChecker) -> {
                 system.addSubtypeConstraint(initialType, expectedType, expectedTypeConstraintPosition)
             }
             system.notFixedTypeVariables.isEmpty() -> return

compiler/testData/diagnostics/tests/controlStructures/ifToAnyDiscriminatingUsages.fir.kt

@@ -27,7 +27,7 @@ fun testResultOfAnonFun2() =
 
 fun testReturnFromAnonFun() =
         run(fun () {
-            return if (true) 42 else println()
+            return <!ARGUMENT_TYPE_MISMATCH, RETURN_TYPE_MISMATCH!>if (true) 42 else println()<!>
         })
 
 fun <!IMPLICIT_NOTHING_RETURN_TYPE!>testReturn1<!>() =

compiler/testData/diagnostics/tests/controlStructures/improperElseInExpression.fir.kt

@@ -29,5 +29,5 @@ fun example() {
         return <!INVALID_IF_AS_EXPRESSION!>if<!> (true) true
     }
 
-    return if (true) true else {}
+    return <!RETURN_TYPE_MISMATCH!>if (true) true else {}<!>
 }