[flang] Modify an IO format error message

[vdonaldson]

Summary

F18 constraint C1308 is:

For the G edit descriptor, e shall not be specified if w is zero.

For an edit descriptor such as 'G0.2E4', change the error message from:

error: Unexpected 'e' in 'G0' edit descriptor

To:

error: A 'G0' edit descriptor must not have an 'e' value

[vdonaldson]

https://reviews.llvm.org/D116622

flang/include/flang/Common/format.h

case TokenKind::G:
       check_r();
       NextToken();
       if (check_w()) {
         if (wValue_ > 0) {
           if (check_d()) { // C1307
             check_e();
           }
         } else if (token_.kind() == TokenKind::Point && check_d() &&
-            token_.kind() == TokenKind::E) {
-          ReportError("Unexpected 'e' in 'G0' edit descriptor"); // C1308
+            token_.kind() == TokenKind::E) { // C1308
+          ReportError("A 'G0' edit descriptor must not have an 'e' value");
           NextToken();
           if (token_.kind() == TokenKind::UnsignedInteger) {
             NextToken();
           }
         }
       }
       break;
     case TokenKind::L:

flang/test/Semantics/io07.f90

 
 7001 format(17G8.1, 17G8.1e3)
 
      !ERROR: Expected 'G' edit descriptor '.d' value
 7101 format(17G8)
 
 8001 format(9G0.5)
 
-     !ERROR: Unexpected 'e' in 'G0' edit descriptor
+     !ERROR: A 'G0' edit descriptor must not have an 'e' value
 8101 format(9(G0.5e1))
 
-     !ERROR: Unexpected 'e' in 'G0' edit descriptor
+     !ERROR: A 'G0' edit descriptor must not have an 'e' value
 8102 format(9(G0.5  E 1))
 end

flang/test/Semantics/io08.f90

! RUN: %python %S/test_errors.py %s %flang_fc1
   write(*,'(A0)')
 
   !ERROR: 'L' edit descriptor 'w' value must be positive
   write(*,'(L0)')
 
   !ERROR: Expected 'G' edit descriptor '.d' value
   write(*,'(G4)')
 
-  !ERROR: Unexpected 'e' in 'G0' edit descriptor
+  !ERROR: A 'G0' edit descriptor must not have an 'e' value
   write(*,'(G0.8e)')
 
-  !ERROR: Unexpected 'e' in 'G0' edit descriptor
+  !ERROR: A 'G0' edit descriptor must not have an 'e' value
   write(*,'(G0.8e2)')
 
   !ERROR: Kind parameter '_' character in format expression
   write(*,'(I5_4)')
 
   !ERROR: Kind parameter '_' character in format expression
   write(*,'(5_4P)')