ShellPkg/redirection: Insert \xFEFF after converting ASCII to Unicode

ShellPkg/redirection: Insert \xFEFF after converting ASCII to Unicode

When "<a" is used to redirect ASCII file to an application, Shell
core reads the ASCII file and converts the ASCII to Unicode as the
input source of the application.
But per Shell spec, the input source should have \xFEFF to indicate
it's a Unicode stream.
The patch adds the missing \xFEFF.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>