Basic support of partially specified type arguments though a new underscore operator for type arguments

^KT-13394

compiler/testData/diagnostics/tests/inference/illegalUnderscoredTypeArgument.fir.kt

@@ -0,0 +1,10 @@
+// WITH_RUNTIME
+
+fun <K, T> foo(x: (K) -> T): Pair<K, T> = TODO()
+
+class Foo<K>
+
+fun main() {
+    val x = foo<Int, Foo<_>> { it.toFloat() }
+
+}

compiler/testData/diagnostics/tests/inference/illegalUnderscoredTypeArgument.kt

@@ -0,0 +1,10 @@
+// WITH_RUNTIME
+
+fun <K, T> foo(x: (K) -> T): Pair<K, T> = TODO()
+
+class Foo<K>
+
+fun main() {
+    val x = foo<Int, Foo<<!UNRESOLVED_REFERENCE!>_<!>>> { <!TYPE_MISMATCH!>it.toFloat()<!> }
+
+}

compiler/testData/diagnostics/tests/inference/illegalUnderscoredTypeArgument.txt

@@ -0,0 +1,11 @@
+package
+
+public fun </*0*/ K, /*1*/ T> foo(/*0*/ x: (K) -> T): kotlin.Pair<K, T>
+public fun main(): kotlin.Unit
+
+public final class Foo</*0*/ K> {
+    public constructor Foo</*0*/ K>()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}