Object, COFF: Don't consider AuxFunctionDefinition for getSymbolSize

mingw lies about the size of a function's AuxFunctionDefinition.  Ignore
the field and rely on our heuristic to determine the symbol's size.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221485 91177308-0d34-0410-b5e6-96231b3b80d8