[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 ->  {
+                }
             }
 
         }

compiler/testData/ir/irText/expressions/whenCoercedToUnit.fir.txt

@@ -2,9 +2,9 @@ FILE fqName:<root> fileName:/whenCoercedToUnit.kt
   FUN name:foo visibility:public modality:FINAL <> (x:kotlin.Int) returnType:kotlin.Unit
     VALUE_PARAMETER name:x index:0 type:kotlin.Int
     BLOCK_BODY
-      BLOCK type=kotlin.Int origin=WHEN
+      BLOCK type=kotlin.Unit origin=WHEN
         VAR IR_TEMPORARY_VARIABLE name:tmp0_subject type:kotlin.Int [val]
-        WHEN type=kotlin.Int origin=WHEN
+        WHEN type=kotlin.Unit origin=WHEN
           BRANCH
             if: CALL 'public final fun EQEQ (arg0: kotlin.Any?, arg1: kotlin.Any?): kotlin.Boolean declared in kotlin.internal.ir' type=kotlin.Boolean origin=EQEQ
               arg0: GET_VAR 'val tmp0_subject: kotlin.Int [val] declared in <root>.foo' type=kotlin.Int origin=null

compiler/testData/ir/irText/regressions/kt24114.fir.txt

@@ -11,16 +11,16 @@ FILE fqName:<root> fileName:/kt24114.kt
     BLOCK_BODY
       WHILE label=null origin=WHILE_LOOP
         condition: CONST Boolean type=kotlin.Boolean value=true
-        body: BLOCK type=kotlin.Int origin=WHEN
+        body: BLOCK type=kotlin.Unit origin=WHEN
           VAR IR_TEMPORARY_VARIABLE name:tmp0_subject type:kotlin.Int [val]
-          WHEN type=kotlin.Int origin=WHEN
+          WHEN type=kotlin.Unit origin=WHEN
             BRANCH
               if: CALL 'public final fun EQEQ (arg0: kotlin.Any?, arg1: kotlin.Any?): kotlin.Boolean declared in kotlin.internal.ir' type=kotlin.Boolean origin=EQEQ
                 arg0: GET_VAR 'val tmp0_subject: kotlin.Int [val] declared in <root>.test1' type=kotlin.Int origin=null
                 arg1: CONST Int type=kotlin.Int value=1
-              then: BLOCK type=kotlin.Nothing origin=WHEN
+              then: BLOCK type=kotlin.Unit origin=WHEN
                 VAR IR_TEMPORARY_VARIABLE name:tmp1_subject type:kotlin.Int [val]
-                WHEN type=kotlin.Nothing origin=WHEN
+                WHEN type=kotlin.Unit origin=WHEN
                   BRANCH
                     if: CALL 'public final fun EQEQ (arg0: kotlin.Any?, arg1: kotlin.Any?): kotlin.Boolean declared in kotlin.internal.ir' type=kotlin.Boolean origin=EQEQ
                       arg0: GET_VAR 'val tmp1_subject: kotlin.Int [val] declared in <root>.test1' type=kotlin.Int origin=null
@@ -35,16 +35,16 @@ FILE fqName:<root> fileName:/kt24114.kt
     BLOCK_BODY
       WHILE label=null origin=WHILE_LOOP
         condition: CONST Boolean type=kotlin.Boolean value=true
-        body: BLOCK type=kotlin.Int origin=WHEN
+        body: BLOCK type=kotlin.Unit origin=WHEN
           VAR IR_TEMPORARY_VARIABLE name:tmp2_subject type:kotlin.Int [val]
-          WHEN type=kotlin.Int origin=WHEN
+          WHEN type=kotlin.Unit origin=WHEN
             BRANCH
               if: CALL 'public final fun EQEQ (arg0: kotlin.Any?, arg1: kotlin.Any?): kotlin.Boolean declared in kotlin.internal.ir' type=kotlin.Boolean origin=EQEQ
                 arg0: GET_VAR 'val tmp2_subject: kotlin.Int [val] declared in <root>.test2' type=kotlin.Int origin=null
                 arg1: CONST Int type=kotlin.Int value=1
-              then: BLOCK type=kotlin.Nothing origin=WHEN
+              then: BLOCK type=kotlin.Unit origin=WHEN
                 VAR IR_TEMPORARY_VARIABLE name:tmp3_subject type:kotlin.Int [val]
-                WHEN type=kotlin.Nothing origin=WHEN
+                WHEN type=kotlin.Unit origin=WHEN
                   BRANCH
                     if: CALL 'public final fun EQEQ (arg0: kotlin.Any?, arg1: kotlin.Any?): kotlin.Boolean declared in kotlin.internal.ir' type=kotlin.Boolean origin=EQEQ
                       arg0: GET_VAR 'val tmp3_subject: kotlin.Int [val] declared in <root>.test2' type=kotlin.Int origin=null