[clang] [llvm] [win][x64] Unwind v2 3/n: Add support for requiring unwind v2 to be used (equivalent to MSVC's /d2epilogunwindrequirev2) (PR #143577)

[Daniel Paoliello via llvm-commits]

dpaoliello wrote:

I'm happy to change these back to `llvm_unreachable` or `reportFatalInternalError`. Is it possible for something other than LLVM to emit the prolog and epilog, like hand-written assembly or a frontend (e.g., for a custom calling convention).

I'll change the too many epilog error to add some guidance (only reasonable thing to do is to manually split the function).

https://github.com/llvm/llvm-project/pull/143577