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/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)