[PECOFF] WriterPECOFF: Rename getCharacteristics -> computeCharacteristics.

The base class has a member function with the same name. We should avoid it
being shadowed.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@196992 91177308-0d34-0410-b5e6-96231b3b80d8