MdePkg: BaseOrderedCollectionRedBlackTreeLib: improve coding style

- The edk2 coding style prefers each variable declaration to stand on
  its own line.
- Internal linkage (ie. STATIC) functions have caused problems with
  source level debugging before, so we generally avoid STATIC in MdePkg.
- Even forward declarations of functions should carry full comment
  blocks.
- Nullity checks in controlling expressions should be spelled out
  explicitly, as (Ptr != NULL).

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/MdePkg@15843 6f19259b-4bc3-4df7-8a09-765794883524