[FIR] Introduce ConeStubType to have subtyping for non-fixed variables

Currently, it's needed after changes in d7b47108f70a107818a04c8b8db33bfbf7c7e590

 Later it'll be also used for builder-inference

compiler/fir/cones/src/org/jetbrains/kotlin/fir/types/ConeTypes.kt

@@ -8,6 +8,7 @@ package org.jetbrains.kotlin.fir.types
 import org.jetbrains.kotlin.fir.symbols.ConeClassLikeLookupTag
 import org.jetbrains.kotlin.fir.symbols.ConeClassifierLookupTag
 import org.jetbrains.kotlin.name.ClassId
+import org.jetbrains.kotlin.name.Name
 import org.jetbrains.kotlin.types.model.*
 
 sealed class ConeKotlinTypeProjection : TypeArgumentMarker {
@@ -193,3 +194,17 @@ class ConeIntersectionType(
 fun ConeIntersectionType.mapTypes(func: (ConeKotlinType) -> ConeKotlinType): ConeIntersectionType {
     return ConeIntersectionType(intersectedTypes.map(func))
 }
+
+class ConeStubType(val variable: ConeTypeVariable, override val nullability: ConeNullability) : StubTypeMarker, ConeKotlinType() {
+    override val typeArguments: Array<out ConeKotlinTypeProjection>
+        get() = emptyArray()
+}
+
+open class ConeTypeVariable(name: String) : TypeVariableMarker {
+    val typeConstructor = ConeTypeVariableTypeConstructor(name)
+    val defaultType = ConeTypeVariableType(ConeNullability.NOT_NULL, typeConstructor)
+}
+
+class ConeTypeVariableTypeConstructor(val debugName: String) : ConeClassifierLookupTag(), TypeVariableTypeConstructorMarker {
+    override val name: Name get() = Name.identifier(debugName)
+}

compiler/fir/cones/src/org/jetbrains/kotlin/fir/types/TypeRenderer.kt

@@ -46,5 +46,6 @@ fun ConeKotlinType.render(): String {
                 postfix = ")"
             )
         }
+        is ConeStubType -> "stub type: $variable"
     } + nullabilitySuffix
 }