MdePkg-BaseLib: Fix PathCleanUpDirectories() issue with "\\..\\.."

MdePkg-BaseLib: Fix PathCleanUpDirectories() issue with "\\..\\.."

Replace multiple, consecutive "\" characters prior to other processing
involving "\" characters.  This fixes an issue where "\\..\\..",
"//..//..", and similar input paths are not cleaned properly.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jim Dailey <jim_dailey@dell.com>
Reviewed-by: Ruiyu Ni <Ruiyu.ni@Intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>