[PATCH] unsigned count() for ADT

[Yaron Keren]

I modified size_type for BitVector back from size_t to unsigned. 
The code caused a compile building failure on LP64 systems where sizeof(size_t)==sizeof(long) with template deduction in line 560. The failure could be fixed otherwise but anyhow I don't think that we have the need to store that many bits. 

Rereading BitVector.h, if size were unsigned long instead of unsigned then members Size, Capacity should also be unsigned long.

http://reviews.llvm.org/D4018