MdePkg/BaseLib: re-specify Base64Decode(), and add temporary stub impl

MdePkg/BaseLib: re-specify Base64Decode(), and add temporary stub impl

Rewrite Base64Decode() from scratch, due to reasons listed in the second
reference below.

As first step, redo the interface contract, and replace the current
implementation with a stub that asserts FALSE, then fails.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Marvin Häuser <mhaeuser@outlook.de>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: Zhichao Gao <zhichao.gao@intel.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1891
Ref: http://mid.mail-archive.com/c495bd0b-ea4d-7206-8a4f-a7149760d19a@redhat.com
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>