Constructors represented "as in Java".

TODO: extract nested initializers somehow.

compiler/testData/ir/irText/classes/primaryConstructorWithSuperConstructorCall.kt

@@ -0,0 +1,10 @@
+open class Base
+
+class TestImplicitPrimaryConstructor : Base()
+
+class TestExplicitPrimaryConstructor() : Base()
+
+class TestWithDelegatingConstructor(val x: Int, val y: Int) : Base() {
+    constructor(x: Int) : this(x, 0)
+}
+