[flang][runtime] Trim FORMATs echoed to error messages

[klausler]

Summary

Since dynamic FORMAT strings usually come from blank-padded fixed-length
CHARACTER variables, trim leading and trailing blanks from them when they
are echoed to error messages for better readability.

flang/runtime/format.h

private:
   int CueUpNextDataEdit(Context &, bool stop = false);
 
   static constexpr CharType Capitalize(CharType ch) {
     return ch >= 'a' && ch <= 'z' ? ch + 'A' - 'a' : ch;
   }
 
   void ReportBadFormat(Context &context, const char *msg, int offset) const {
     if constexpr (std::is_same_v<CharType, char>) {
+      // Echo the bad format in the error message, but trim any leading or
+      // trailing spaces.
+      int firstNonBlank{0};
+      while (firstNonBlank < formatLength_ && format_[firstNonBlank] == ' ') {
+        ++firstNonBlank;
+      }
+      int lastNonBlank{formatLength_ - 1};
+      while (lastNonBlank > firstNonBlank && format_[lastNonBlank] == ' ') {
+        --lastNonBlank;
+      }
+      if (firstNonBlank <= lastNonBlank) {
         context.SignalError(IostatErrorInFormat,
-          "%s; at offset %d in format '%s'", msg, offset, format_);
-    } else {
-      context.SignalError(IostatErrorInFormat, "%s; at offset %d", msg, offset);
+            "%s; at offset %d in format '%.*s'", msg, offset,
+            lastNonBlank - firstNonBlank + 1, format_ + firstNonBlank);
+        return;
       }
     }
+    context.SignalError(IostatErrorInFormat, "%s; at offset %d", msg, offset);
+  }
 
   // Data members are arranged and typed so as to reduce size.
   // This structure may be allocated in stack space loaned by the
   // user program for internal I/O.
   const std::uint8_t maxHeight_{maxMaxHeight};
   std::uint8_t height_{0};
   const CharType *format_{nullptr};
   int formatLength_{0};
   int offset_{0}; // next item is at format_[offset_]
 
   // must be last, may be incomplete
   Iteration stack_[maxMaxHeight];
 };
 } // namespace Fortran::runtime::io
 #endif // FORTRAN_RUNTIME_FORMAT_H_