Since most Windows users do not have a C compiler/linker, GHC has long shipped with its own Clang toolchain, derived from msys2 packaging. The last several releases we did not upgrade this toolchain but for 9.14 we felt it was time to do so.
Even in the best of times such upgrades will typically have effects on GHC. In this particular case we were hit with Microsoft’s restructuring of its standard libraries into so-called API sets as well as changes in the archive format emitted by Clang. Both of these required changes in GHC and its runtime system linker to accommodate.