Rename the VOID type to INVALID and clean up obsolete comments.

The VOID type isn't used for anything resembling what "void" means in C,
so rename it to INVALID to avoid confusion.

lib/codegen/meta-python/cdsl/types.py

@@ -179,7 +179,7 @@ class SpecialType(ValueType):
     def __init__(self, name, membytes, doc):
         # type: (str, int, str) -> None
         super(SpecialType, self).__init__(name, membytes, doc)
-        # Assign numbers starting from 1. (0 is VOID)
+        # Assign numbers starting from 1. (0 is INVALID)
         ValueType.all_special_types.append(self)
         self.number = len(ValueType.all_special_types)
         assert self.number < LANE_BASE, 'Too many special types'