MdePkg/Include/Library: Undefine _ASSERT() if already defined

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



When unit testing is enabled, make sure _ASSERT() is not already
defined by the host environment before defining _ASSERT().  This
avoids conflicts with VS20xx builds of GoogleTest based unit tests.

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>