[Blocking] Fix #3840: Clean up logic for parsing tree_method parameter (#3849)

* Clean up logic for converting tree_method to updater sequence

* Use C++11 enum class for extra safety

Compiler will give warnings if switch statements don't handle all
possible values of C++11 enum class.

Also allow enum class to be used as DMLC parameter.

* Fix compiler error + lint

* Address reviewer comment

* Better docstring for DECLARE_FIELD_ENUM_CLASS

* Fix lint

* Add C++ test to see if tree_method is recognized

* Fix clang-tidy error

* Add test_learner.h to R package

* Update comments

* Fix lint error

tests/cpp/common/test_enum_class_param.cc

@@ -0,0 +1,55 @@
+#include "../../../src/common/enum_class_param.h"
+#include <dmlc/parameter.h>
+#include <gtest/gtest.h>
+
+enum class Foo : int {
+  kBar = 0, kFrog = 1, kCat = 2, kDog = 3
+};
+
+DECLARE_FIELD_ENUM_CLASS(Foo);
+
+struct MyParam : dmlc::Parameter<MyParam> {
+  Foo foo;
+  int bar;
+  DMLC_DECLARE_PARAMETER(MyParam) {
+    DMLC_DECLARE_FIELD(foo)
+      .set_default(Foo::kBar)
+      .add_enum("bar", Foo::kBar)
+      .add_enum("frog", Foo::kFrog)
+      .add_enum("cat", Foo::kCat)
+      .add_enum("dog", Foo::kDog);
+    DMLC_DECLARE_FIELD(bar)
+      .set_default(-1);
+  }
+};
+
+DMLC_REGISTER_PARAMETER(MyParam);
+
+TEST(EnumClassParam, Basic) {
+  MyParam param;
+  std::map<std::string, std::string> kwargs{
+    {"foo", "frog"}, {"bar", "10"}
+  };
+  // try initializing
+  param.Init(kwargs);
+  ASSERT_EQ(param.foo, Foo::kFrog);
+  ASSERT_EQ(param.bar, 10);
+
+  // try all possible enum values
+  kwargs["foo"] = "bar";
+  param.Init(kwargs);
+  ASSERT_EQ(param.foo, Foo::kBar);
+  kwargs["foo"] = "frog";
+  param.Init(kwargs);
+  ASSERT_EQ(param.foo, Foo::kFrog);
+  kwargs["foo"] = "cat";
+  param.Init(kwargs);
+  ASSERT_EQ(param.foo, Foo::kCat);
+  kwargs["foo"] = "dog";
+  param.Init(kwargs);
+  ASSERT_EQ(param.foo, Foo::kDog);
+
+  // try setting non-existent enum value
+  kwargs["foo"] = "human";
+  ASSERT_THROW(param.Init(kwargs), dmlc::ParamError);
+}