[Support] Use llvm_unreachable instead of LLVM_BUILTIN_UNREACHABLE internal define

Author: RKSimon

llvm/lib/Support/NativeFormatting.cpp

@@ -258,5 +258,5 @@ size_t llvm::getDefaultPrecision(FloatStyle Style) {
   case FloatStyle::Percent:
     return 2; // Number of decimal places.
   }
-  LLVM_BUILTIN_UNREACHABLE;
+  llvm_unreachable("Unknown FloatStyle enum");
 }