Add xrOS to silence -Wswitch violations (#71432)

To commit the necessary llvm-project changes to land xrOS support,  xrOS as a triple must be understood to swift. Add minimal support such that CI passes.

Author: cyndyishida

lib/Basic/Platform.cpp

@@ -180,6 +180,7 @@ StringRef swift::getPlatformNameForTriple(const llvm::Triple &triple) {
   case llvm::Triple::CloudABI:
   case llvm::Triple::DragonFly:
   case llvm::Triple::DriverKit:
+  case llvm::Triple::XROS:
   case llvm::Triple::Emscripten:
   case llvm::Triple::Fuchsia:
   case llvm::Triple::KFreeBSD: