[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

   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
 

flang/lib/Parser/prescan.cpp

       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();

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