LLVM source file:

D:\src_builds\llvm-monorepo\llvm\llvm\lib\MC\WinCOFFObjectWriter.cpp

Around line 711, comment out as follows:

    /* Asm.getContext().reportError(Fixup.getLoc(),
                                      Twine("assembler label '") + A.getName() +
                                          "' can not be undefined"); */