Good midnight, everyone!
Now clang/llvm on cygming can be built selfhost with a few patch 
I am trying to build stage2 clang/llvm with -integrated-as.
I met some issues below;
* MC-COFF emits .comm as individual "linkonce" section.
  Thus, the order in .bss must be differ against one assembled by GNU as.
  It might have bugs and I am investigating now.
* Trail alignment shim of each section
  GNU as emits alignment to trail of .text. MC-COFF does not.
* MC-COFF does relaxation more strongly
  I saw a few cases, MC-COFF emits disp8 for branch (although GNU as
emits disp32).
  At a glance, It might be correct.
  I have not met regression yet.
* (Investigating)
  I saw a few cases; MC-COFF emits quite different instruction sequences.
  It might be my mistake, still in investigating.
I will file one for serious issues.
have a nice hacking! ...Takumi