[FIR] Add synthetic else branch to when if it's missing

This change is needed for correct work of data flow, because for
  non-exhaustive when there is a else branch in terms of control flow
  graph.

This change leads to some errors when when is actually exhaustive, but
  we still add else branch. Those errors will be gone when exhaustive
  checker will be introduced

compiler/fir/psi2fir/src/org/jetbrains/kotlin/fir/builder/RawFirBuilder.kt

@@ -943,6 +943,7 @@ class RawFirBuilder(session: FirSession, val stubMode: Boolean) : BaseFirBuilder
                 if (hasSubject) {
                     subject.bind(this)
                 }
+                var thereIsElseBranch = false
                 for (entry in expression.entries) {
                     val branch = entry.expression.toFirBlock()
                     branches += if (!entry.isElse) {
@@ -960,9 +961,13 @@ class RawFirBuilder(session: FirSession, val stubMode: Boolean) : BaseFirBuilder
                             FirWhenBranchImpl(entry, firCondition, branch)
                         }
                     } else {
+                        thereIsElseBranch = true
                         FirWhenBranchImpl(entry, FirElseIfTrueCondition(null), branch)
                     }
                 }
+                if (!thereIsElseBranch) {
+                    branches += FirWhenBranchImpl(null, FirElseIfTrueCondition(null), FirEmptyExpressionBlock())
+                }
             }
         }
 

compiler/fir/resolve/testData/resolve/recursiveCallOnWhenWithSealedClass.txt

@@ -35,6 +35,8 @@ FILE: recursiveCallOnWhenWithSealedClass.kt
                 ($subj$ is R|Maybe.Yeah|) ->  {
                     <Unresolved name: meat>#
                 }
+                else ->  {
+                }
             }
 
         }