MdePkg/BaseLib: AsciiStrToUnicodeStr(S) not handle EASCII properly

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=1262

Current implementation of BaseLib APIs:

AsciiStrToUnicodeStr()
AsciiStrToUnicodeStrS()
AsciiStrnToUnicodeStrS()

do not handle EASCII properly.

More specifically, if the value of ASCII character is larger than 0x7F,
then the converted Unicode character will have all '1's in the higher 8
bits.

An example:
  0xC9 => 0xFFC9 (current implementations)
and it should be:
  0xC9 => 0x00C9

This commit will address this issue.

Cc: Bin.Lain <bin_601@mail2000.com.tw>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

MdePkg/Library/BaseLib/String.c

@@ -1746,7 +1746,7 @@ AsciiStrToUnicodeStr (
 
   ReturnValue = Destination;
   while (*Source != '\0') {
-    *(Destination++) = (CHAR16) *(Source++);
+    *(Destination++) = (CHAR16)(UINT8) *(Source++);
   }
   //
   // End the Destination with a NULL.