Fix generic signature for SAM adapters and constructors

SAM-related code in codegen was using JavaClassDescriptor directly, which has
an erased signature. Create and use a new abstraction SamType which has a full
generic signature of a type which was used in the SAM construct

compiler/testData/codegen/boxWithJava/sam/samConstructorGenericSignature.kt

@@ -0,0 +1,8 @@
+import java.util.Arrays
+
+fun box(): String {
+    val r: JavaClass.Computable<String> = JavaClass.Computable { "OK" }
+    val supertypes = Arrays.toString(r.getClass().getGenericInterfaces())
+    if (supertypes != "[JavaClass.JavaClass\$Computable<java.lang.String>]") return "Fail: $supertypes"
+    return JavaClass.compute(r)!!
+}