MdePkg/BaseLib: Add 2 more safe string functions.

MdePkg/BaseLib: Add 2 more safe string functions.

Add UnicodeStrToAsciiStrS() and AsciiStrToUnicodeStrS() API.
These 2 safe version APIs are used to replace UnicodeStrToAsciiStr() and
AsciiStrToUnicodeStr() API.

The safe version string convert APIs use similar check as StrCpyS().

Cc: Liming Gao <Liming.Gao@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jiewen Yao <Jiewen.Yao@intel.com>
Reviewed-by: Liming Gao <Liming.Gao@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>