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.java

@@ -0,0 +1,9 @@
+class JavaClass {
+    interface Computable<T> {
+        T compute();
+    }
+
+    static <T> T compute(Computable<T> computable) {
+        return computable.compute();
+    }
+}