[flang][runtime] Allow OPEN(n,ENCODING=) to change the encoding

[klausler]

Summary

OPEN statements can be used to change some, but not all, attributes
of units that have already been opened. The I/O runtime library
wasn't allowing ENCODING= to be changed. Every other Fortran compiler
permits this usage, and it's safe and useful, so allow it.
(Otherwise there's no good way to ensure that the preconnected
unit 6 is in UTF-8 mode.)

flang/docs/Extensions.md

 
 ## De Facto Standard Features
 
 * `EXTENDS_TYPE_OF()` returns `.TRUE.` if both of its arguments have the
   same type, a case that is technically implementation-defined.
+
+* `ENCODING=` is not in the list of changeable modes on an I/O unit,
+  but every Fortran compiler allows the encoding to be changed on an
+  open unit.

flang/runtime/io-api.cpp

     return false;
   } else if (open->completedOperation()) {
     io.GetIoErrorHandler().Crash(
         "SetEncoding() called after GetNewUnit() for an OPEN statement");
   }
-  bool isUTF8{false};
+  // Allow the encoding to be changed on an open unit -- it's
+  // useful and safe.
   static const char *keywords[]{"UTF-8", "DEFAULT", nullptr};
   switch (IdentifyValue(keyword, length, keywords)) {
   case 0:
-    isUTF8 = true;
+    open->unit().isUTF8 = true;
     break;
   case 1:
-    isUTF8 = false;
+    open->unit().isUTF8 = false;
     break;
   default:
     open->SignalError(IostatErrorInKeyword, "Invalid ENCODING='%.*s'",
         static_cast<int>(length), keyword);
   }
-  if (isUTF8 != open->unit().isUTF8) {
-    if (open->wasExtant()) {
-      open->SignalError("ENCODING= may not be changed on an open unit");
-    }
-    open->unit().isUTF8 = isUTF8;
-  }
   return true;
 }
 
 bool IONAME(SetForm)(Cookie cookie, const char *keyword, std::size_t length) {
   IoStatementState &io{*cookie};