[flang] Emit portability warning for extension

[klausler]

Summary

f18 accepts statement labels in fixed form source even if they follow
a semicolon -- i.e., they're not in the fixed form's label field.
Emit a warning for such usage.

flang/docs/Extensions.md

* A type-bound procedure binding can be passed as an actual
   argument corresponding to a dummy procedure and can be used as
   the target of a procedure pointer assignment statement.
 * An explicit `INTERFACE` can declare the interface of a
   procedure pointer even if it is not a dummy argument.
 * A `NOPASS` type-bound procedure binding is required by C1529
   to apply only to a scalar data-ref, but most compilers don't
   enforce it and the constraint is not necessary for a correct
   implementation.
+* A label may follow a semicolon in fixed form source.
 
 ### Extensions supported when enabled by options
 
 * C-style backslash escape sequences in quoted CHARACTER literals
   (but not Hollerith) [-fbackslash]
 * Logical abbreviations `.T.`, `.F.`, `.N.`, `.A.`, `.O.`, and `.X.`
   [-flogical-abbreviations]
 * `.XOR.` as a synonym for `.NEQV.` [-fxor-operator]

flang/lib/Parser/prescan.cpp

if ((nch == '=' &&
         (ch == nch &&
             (ch == '/' || ch == ':' || ch == '*' || ch == '#' || ch == '&' ||
                 ch == '|' || ch == '<' || ch == '>')) ||
         (ch == '=' && nch == '>')) {
       // token comprises two characters
       EmitCharAndAdvance(tokens, nch);
     } else if (ch == '/') {
       slashInCurrentStatement_ = true;
+    } else if (ch == ';' && InFixedFormSource()) {
+      SkipSpaces();
+      if (IsDecimalDigit(*at_)) {
+        Say(GetProvenanceRange(at_, at_ + 1),
+            "Label should be in the label field"_port_en_US);
+      }
     }
   }
   tokens.CloseToken();
   return true;
 }
 
 bool Prescanner::ExponentAndKind(TokenSequence &tokens) {
   char ed{ToLowerCaseLetter(*at_)};

flang/test/Parser/bad-label.f

+! RUN: %flang_fc1 -fsyntax-only -pedantic %s 2>&1 | FileCheck %s
+!CHECK: portability: Label should be in the label field
+      goto 1;  1 continue
+      end