Fix bug where WinCOFFObjectWriter would assume starting from an empty output.

Starting on an input stream that is not at offset 0 would trigger the
assert in WinCOFFObjectWriter.cpp:1065:

  assert(getStream().tell() <= (*i)->Header.PointerToRawData &&
               "Section::PointerToRawData is insane!");

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253464 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed