Expand ConeTypeContext, implement FIR type inference & related checkers

compiler/fir/tree/src/org/jetbrains/kotlin/fir/FirRenderer.kt

@@ -34,6 +34,8 @@ fun FirElement.render(): String = buildString { this@render.accept(FirRenderer(t
 
 fun ConeKotlinType.render(): String {
     return when (this) {
+        is ConeTypeVariableType -> "TypeVariable(${this.lookupTag.name})"
+        is ConeDefinitelyNotNullType -> "${original.render()}!"
         is ConeKotlinErrorType -> "error: $reason"
         is ConeClassErrorType -> "class error: $reason"
         is ConeCapturedType -> "captured type: lowerType = ${lowerType?.render()}"

compiler/fir/tree/src/org/jetbrains/kotlin/fir/references/FirResolvedCallableReferenceImpl.kt

@@ -12,7 +12,7 @@ import org.jetbrains.kotlin.fir.FirSession
 import org.jetbrains.kotlin.fir.symbols.ConeCallableSymbol
 import org.jetbrains.kotlin.name.Name
 
-class FirResolvedCallableReferenceImpl(
+open class FirResolvedCallableReferenceImpl(
     session: FirSession,
     psi: PsiElement?,
     override val name: Name,

compiler/fir/tree/src/org/jetbrains/kotlin/fir/types/impl/FirTypePlaceholderProjection.kt

@@ -0,0 +1,17 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.fir.types.impl
+
+import com.intellij.psi.PsiElement
+import org.jetbrains.kotlin.fir.FirSession
+import org.jetbrains.kotlin.fir.types.FirTypeProjection
+
+object FirTypePlaceholderProjection : FirTypeProjection {
+    override val psi: PsiElement?
+        get() = null // TODO
+    override val session: FirSession
+        get() = error("ahahahhahah")
+}