[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

Makefile

@@ -250,6 +250,8 @@ Rpack: clean_all
 	cp -r src xgboost/src/src
 	cp -r include xgboost/src/include
 	cp -r amalgamation xgboost/src/amalgamation
+	mkdir -p xgboost/src/tests/cpp
+	cp tests/cpp/test_learner.h xgboost/src/tests/cpp
 	mkdir -p xgboost/src/rabit
 	cp -r rabit/include xgboost/src/rabit/include
 	cp -r rabit/src xgboost/src/rabit/src