[LL] retrieve fir from generated property of desugaring ++ operator

^ KTIJ-24385
Temp property to store receiver is generated for `a.b++` expression.
If this property's psi corresponds to receiver expr, then FirProperty
would be found by mapper if receiver is requested.
It works unexpectedly, because FirProperty is normally not expected by expression.
This change set fake sources for generated FirProperty, so it won't be found
by source psi

analysis/low-level-api-fir/testdata/getOrBuildFir/qualifiedExpressions/qualifiedPartOfQualifiedCallUnresolved.kt

@@ -0,0 +1,5 @@
+class FF {
+    fun handleLeftBracketInFragment() {
+        <expr>peek()</expr>.braceBalance++
+    }
+}

analysis/low-level-api-fir/testdata/getOrBuildFir/qualifiedExpressions/qualifiedPartOfQualifiedCallUnresolved.txt

@@ -0,0 +1,6 @@
+KT element: KtCallExpression
+FIR element: FirFunctionCallImpl
+FIR source kind: KtRealSourceElementKind
+
+FIR element rendered:
+<Unresolved name: peek>#()