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/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