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

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()