WinCOFFObjectWriter should not access WinCOFFWriter, which is supposed to be internal

[MaskRay]

@HaohaiWen @KanRobert https://reviews.llvm.org/D152340 split WinCOFFObjectWriter into WinCOFFObjectWriter and WinCOFFWriter as a prerequisite to implement split DWARF.

WinCOFFObjectWriter is supposed to be public, accessible by MCWinCOFFStreamer.cpp, while WinCOFFWriter should remain internal. As such, WinCOFFObjectWriter should not hold unique_ptr<WinCOFFWriter> instances.

The relationship should be made similar to ELFObjectWriter and ELFWriter.

[MaskRay]

I exported WinCOFFObjectWriter in commit 9539a77 to clean up MCAssembler internals, but I'll not have time to clean up WinCOFFWriter. Perhaps either of you might help? :)

[HaohaiWen]

I'll take a look whether it is possible to delay actions performed by executePostLayoutBinding and recordRelocation to writeObject.
If that is possible, we can create a temp WinCOFFWriter in writeObject just like ELF did.

[HaohaiWen]

Unlike ELF, relocations of COFF are embedded in normal section. Currently implementation collect sections/symbols in executePostLayoutBinding and then assign relocation to corresponding section in recordRelocation. I'm not sure whether its safe to delay check in recordRelocation. It's not cheap to refactor it to be ELFWriter like.
Since WinCOFFWriter is expected to be internal. I'd like to make it private nested class of WinCOFFObjectWriter.

[KanRobert]

@MaskRay skan is not me... My github account is @KanRobert

[MaskRay]

@MaskRay skan is not me... My github account is @KanRobert

Sorry! I knew your username for the occasional PRes but I somehow made the mistake when creating the issue...