[FE] Substitute fixed type variables with inferred stub types

Actually, a type variable might be fixed into a stub type. Such stub type should be substituted before sub calls completion

^KT-51988 Fixed

compiler/testData/codegen/box/inference/builderInference/kt51988.kt

@@ -0,0 +1,37 @@
+// WITH_STDLIB
+// IGNORE_BACKEND_FIR: JVM_IR
+
+import kotlin.experimental.ExperimentalTypeInference
+
+@OptIn(ExperimentalTypeInference::class)
+fun <AB1, BB1> build(@BuilderInference block: BuilderScope<AB1>.() -> BB1): ResultProvider<AB1, BB1> = object : ResultProvider<AB1, BB1> {
+    override fun provideResult(): AB1 = "OK" as AB1
+}
+
+@OptIn(ExperimentalTypeInference::class)
+fun <AB2, BB2> build2(@BuilderInference block: BuilderScope<AB2>.() -> BB2): ResultProvider<AB2, BB2> = object : ResultProvider<AB2, BB2> {
+    override fun provideResult(): AB2 = "OK" as AB2
+}
+
+interface BuilderScope<BS> {
+    fun <B1> getResult(result: ResultProvider<BS, B1>): B1
+    fun <B2> getResult2(result: ResultProvider<BS, B2>): B2
+}
+
+interface ResultProvider<AR, BR> {
+    fun provideResult(): AR
+}
+
+val resultProvider: ResultProvider<Any, Any> = object : ResultProvider<Any, Any> {
+    override fun provideResult(): Any = "NOK"
+}
+
+val result = build {
+    getResult(build2 {
+        getResult2(resultProvider)
+    })
+}
+
+fun box(): String {
+    return result.provideResult().toString()
+}