Embedded: enable *-unknown-windows-coff tripels in Platform.cpp (#…

…72217)

This allows the frontend to proceed when building with `-target i686-unknown-windows-coff -enable-experimental-feature Embedded` arguments.

lib/Basic/Platform.cpp

@@ -225,7 +225,7 @@ StringRef swift::getPlatformNameForTriple(const llvm::Triple &triple) {
  case llvm::Triple::Itanium:
  return "windows";
  default:
- llvm_unreachable("unsupported Windows environment");
+ return "none";
  }
  case llvm::Triple::PS4:
  return "ps4";