BaseTools: fix imports

1 - Some of these imports are cascaded from another file. Import them locally.
2 - Some of these imports are not used. Remove them.
3 - Some of these were missing the namespace used to import them.

These changes facilitate optimization of BaseTools:
https://bugzilla.tianocore.org/show_bug.cgi?id=42

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>

BaseTools/Source/Python/AutoGen/GenC.py

@@ -18,7 +18,7 @@ import string
 import collections
 import struct
 from Common import EdkLogger
-
+from Common import GlobalData
 from Common.BuildToolError import *
 from Common.DataType import *
 from Common.Misc import *