[flang] add a static assert in CheckUnitNumberInRangeImpl

[jeanPerier]

Summary

Add a check that CheckUnitNumberInRangeImpl is not needlessly instantiated.

flang/runtime/io-api.cpp

enum Iostat IONAME(EndIoStatement)(Cookie cookie) {
   IoStatementState &io{*cookie};
   return static_cast<enum Iostat>(io.EndIoStatement());
 }
 
 template <typename INT>
 static enum Iostat CheckUnitNumberInRangeImpl(INT unit, bool handleError,
     char *ioMsg, std::size_t ioMsgLength, const char *sourceFile,
     int sourceLine) {
+  static_assert(sizeof(INT) >= sizeof(ExternalUnit),
+      "only intended to be used when the INT to ExternalUnit conversion is "
+      "narrowing");
   if (unit != static_cast<ExternalUnit>(unit)) {
     Terminator oom{sourceFile, sourceLine};
     IoErrorHandler errorHandler{oom};
     if (handleError) {
       errorHandler.HasIoStat();
       if (ioMsg) {
         errorHandler.HasIoMsg();
       }