Object/COFF: change data type of SymbolNumber from int16 to uint16.

Microsoft PE/COFF Spec clearly states that the field is of signed interger
type. However, in reality, it's unsigned. If cl.exe needs to create a large
number of sections for COMDAT sections, it will just create more than 32768
sections. Handling large section number as negative number is not correct.
I think this is a spec bug.

Differential Revision: http://llvm-reviews.chandlerc.com/D3088