KT-66411 [AA] Add fake source kind for single expression block around array assignments

A wrapper block was introduced as a part of fixing KT-59748, but was
assigned a real source, which had `getOrBuildFir` confused because
of the incorrectly built `KtToFirMapping`

It is relevant for:
- `if`, `when` expressions with an assignment as a single expression
- Kotlin code fragments,
when the assignment is being analysed as a single expression
in the fragment

^KT-66411 Fixed

analysis/analysis-api/testData/referenceResolve/arrayAccess/set.descriptors.txt

@@ -1,8 +0,0 @@
-<caret> resolved to:
-0: (in kotlin.collections.MutableList) operator fun set(index: kotlin.Int, element: E): E
-
-<caret_singleIfStatement> resolved to:
-0: (in kotlin.collections.MutableList) operator fun set(index: kotlin.Int, element: E): E
-
-<caret_singleWhenStatement> resolved to:
-0: (in kotlin.collections.MutableList) operator fun set(index: kotlin.Int, element: E): E

analysis/analysis-api/testData/referenceResolve/arrayAccess/set.txt

@@ -2,7 +2,7 @@
 0: (in kotlin.collections.MutableList) operator fun set(index: kotlin.Int, element: E): E
 
 <caret_singleIfStatement> resolved to:
-Nothing (Unresolved reference)
+0: (in kotlin.collections.MutableList) operator fun set(index: kotlin.Int, element: E): E
 
 <caret_singleWhenStatement> resolved to:
-Nothing (Unresolved reference)
+0: (in kotlin.collections.MutableList) operator fun set(index: kotlin.Int, element: E): E

analysis/analysis-api/testData/referenceResolve/codeFragment/expressionCodeFragment/ArrayAssignmentOperator.txt

@@ -1,2 +1,2 @@
 Resolved to:
-Nothing (Unresolved reference)
+0: (in MyList) operator fun set(index: kotlin.Int, value: kotlin.String)

analysis/analysis-api/testData/referenceResolve/codeFragment/expressionCodeFragment/ArrayPlusAssignmentOperator.txt

@@ -1,2 +1,2 @@
 Resolved to:
-Nothing (Unresolved reference)
+0: (in MyList) operator fun set(index: kotlin.Int, value: kotlin.String)

analysis/low-level-api-fir/testData/getOrBuildFir/calls/arrayAssignOperatorAsSingleExpressionInIf.txt

@@ -1,12 +1,9 @@
 KT element: KtBinaryExpression
-FIR element: FirBlockImpl
+FIR element: FirFunctionCallImpl
 FIR source kind: KtRealSourceElementKind
 
 FIR element rendered:
- {
-    R|<local>/foo|.R|/Foo.set|(Int(10), String())
-    Unit
-}
+R|<local>/foo|.R|/Foo.set|(Int(10), String())
 
 FIR FILE:
 FILE: [ResolvedTo(IMPORTS)] arrayAssignOperatorAsSingleExpressionInIf.kt

analysis/low-level-api-fir/testData/getOrBuildFir/calls/arrayAssignOperatorAsSingleExpressionInWhen.txt

@@ -1,12 +1,9 @@
 KT element: KtBinaryExpression
-FIR element: FirBlockImpl
+FIR element: FirFunctionCallImpl
 FIR source kind: KtRealSourceElementKind
 
 FIR element rendered:
- {
-    R|<local>/foo|.R|/Foo.set|(Int(10), String())
-    Unit
-}
+R|<local>/foo|.R|/Foo.set|(Int(10), String())
 
 FIR FILE:
 FILE: [ResolvedTo(IMPORTS)] arrayAssignOperatorAsSingleExpressionInWhen.kt

compiler/fir/raw-fir/raw-fir.common/src/org/jetbrains/kotlin/fir/builder/AbstractRawFirBuilder.kt

@@ -835,7 +835,7 @@ abstract class AbstractRawFirBuilder<T>(val baseSession: FirSession, val context
                 if (operation == FirOperation.ASSIGN) {
                     val result = unwrappedLhs.convert()
                     result.replaceAnnotations(result.annotations.smartPlus(annotations))
-                    source = result.source
+                    source = result.source?.fakeElement(KtFakeSourceElementKind.IndexedAssignmentCoercionBlock)
                     statements += result.pullUpSafeCallIfNecessary()
                 } else {
                     val receiver = unwrappedLhs.convert()
@@ -846,13 +846,13 @@ abstract class AbstractRawFirBuilder<T>(val baseSession: FirSession, val context
                                 receiver.selector as FirExpression, baseSource, arrayAccessSource, operation, annotations, rhsAST, convert
                             )
                         )
-                        source = receiver.source
+                        source = receiver.source?.fakeElement(KtFakeSourceElementKind.IndexedAssignmentCoercionBlock)
                         statements += receiver
                     } else {
                         val augmentedArraySetCall = generateAugmentedArraySetCall(
                             receiver, baseSource, arrayAccessSource, operation, annotations, rhsAST, convert
                         )
-                        source = augmentedArraySetCall.source
+                        source = augmentedArraySetCall.source?.fakeElement(KtFakeSourceElementKind.IndexedAssignmentCoercionBlock)
                         statements += augmentedArraySetCall
                     }
                 }

compiler/frontend.common/src/org/jetbrains/kotlin/KtSourceElement.kt

@@ -144,6 +144,10 @@ sealed class KtFakeSourceElementKind(final override val shouldSkipErrorTypeRepor
     // with a fake sources for the block which refers to the wrapped expression
     object SingleExpressionBlock : KtFakeSourceElementKind()
 
+    // this source is used for a single fake block created for indexed assignments expression,
+    // see ImplicitUnit.IndexedAssignmentCoercion
+    object IndexedAssignmentCoercionBlock : KtFakeSourceElementKind()
+
     // Contract statements are wrapped in a special block to be reused between a contract FIR and a function body.
     object ContractBlock : KtFakeSourceElementKind()