MdePkg/Include: Update Base.h to improve C++ compatibility

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=4134



* Map NULL to nullptr or __null when c++ compiler is used.
* Map STATIC_ASSERT to static_assert when a c++ compiler is used.
* Typecast RETURN_SUCCESS to type RETURN_STATUS to match type used
  by all return error/warning status codes.  C++ has stricter type
  checking and found this inconsistency.

Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Zhiguang Liu <zhiguang.liu@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>