[FIR] Use a tree with the proper root node

This commit ensures all the conversions inside the lambda expression
use the correct tree reference.

Otherwise, the problem arises in the
`toFirSourceElement()` call: if called form inside the
ExpressionsConverter, everything is OK, but calling it from inside
DeclarationsConverter will result in the wrong source text range.
This happens, because during the lambda expressions conversion a custom
ExpressionsConverter is created with the `lambdaTree` tree, but the
DeclarationsConverter continues to use the previous tree.

compiler/fir/raw-fir/light-tree2fir/src/org/jetbrains/kotlin/fir/lightTree/converter/DeclarationsConverter.kt

@@ -58,7 +58,7 @@ import org.jetbrains.kotlin.utils.addToStdlib.runUnless
 
 class DeclarationsConverter(
     session: FirSession,
-    private val baseScopeProvider: FirScopeProvider,
+    internal val baseScopeProvider: FirScopeProvider,
     tree: FlyweightCapableTreeStructure<LighterASTNode>,
     @set:PrivateForInline override var offset: Int = 0,
     context: Context<LighterASTNode> = Context(),

compiler/fir/raw-fir/light-tree2fir/src/org/jetbrains/kotlin/fir/lightTree/converter/ExpressionsConverter.kt

@@ -74,10 +74,12 @@ class ExpressionsConverter(
                 val lambdaTree = LightTree2Fir.buildLightTreeLambdaExpression(expression.asText)
                 // Pass on label user to the lambda root
                 context.forwardLabelUsagePermission(expression, lambdaTree.root)
-                declarationsConverter.withOffset(offset + expression.startOffset) {
-                    ExpressionsConverter(baseSession, lambdaTree, declarationsConverter, context)
-                        .convertLambdaExpression(lambdaTree.root)
-                }
+                val lambdaDeclarationsConverter = DeclarationsConverter(
+                    baseSession, declarationsConverter.baseScopeProvider, lambdaTree,
+                    offset = offset + expression.startOffset, context
+                )
+                ExpressionsConverter(baseSession, lambdaTree, lambdaDeclarationsConverter, context)
+                    .convertLambdaExpression(lambdaTree.root)
             }
             BINARY_EXPRESSION -> convertBinaryExpression(expression)
             BINARY_WITH_TYPE -> convertBinaryWithTypeRHSExpression(expression) {