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/adapters/genericSignature.kt

@@ -0,0 +1,5 @@
+fun box(): String {
+    val supertypes = JavaClass.foo { a, b -> a.compareTo(b) }
+    if (supertypes != "[java.util.Comparator<java.lang.String>]") return "Fail: $supertypes"
+    return "OK"
+}