[Flang][OpenMP] Fix comments that should not be Sentinels on fixed format. (#68911)

Fixes #68653

Author: Z572

flang/lib/Parser/prescan.cpp

@@ -1177,16 +1177,14 @@ Prescanner::IsFixedFormCompilerDirectiveLine(const char *start) const {
   char sentinel[5], *sp{sentinel};
   int column{2};
   for (; column < 6; ++column, ++p) {
-    if (*p != ' ') {
-      if (*p == '\n' || *p == '\t') {
-        break;
-      }
-      if (sp == sentinel + 1 && sentinel[0] == '$' && IsDecimalDigit(*p)) {
-        // OpenMP conditional compilation line: leave the label alone
-        break;
-      }
-      *sp++ = ToLowerCaseLetter(*p);
+    if (*p == ' ' || *p == '\n' || *p == '\t') {
+      break;
+    }
+    if (sp == sentinel + 1 && sentinel[0] == '$' && IsDecimalDigit(*p)) {
+      // OpenMP conditional compilation line: leave the label alone
+      break;
     }
+    *sp++ = ToLowerCaseLetter(*p);
   }
   if (column == 6) {
     if (*p == ' ' || *p == '\t' || *p == '0') {

flang/test/Parser/OpenMP/sentinels.f

@@ -0,0 +1,42 @@
+! RUN: %flang_fc1 -fopenmp -E %s | FileCheck %s
+! CHECK:      program main
+! CHECK:       interface
+! CHECK:        subroutine sub(a, b)
+! CHECK:!dir$ ignore_tkr a
+! CHECK:!dir$ ignore_tkr b
+! CHECK:          real(4):: a, b
+! CHECK:        end subroutine
+! CHECK:       end interface
+! CHECK:      PRINT *, "Is '    '"
+! CHECK:  123 PRINT *, "Is '123 '"
+
+!@cuf subroutine atcuf;
+      program main
+       interface
+        subroutine sub(a, b)
+!dir$ ignore_tkr a
+!dir$ ignore_tkr
+!dir$+ b
+          real(4):: a, b
+        end subroutine
+       end interface
+!
+!	comment line
+!@fp  PRINT *, "This is a comment line"
+!@f p PRINT *, "This is a comment line"
+!$    PRINT *, "Is '    '"
+!$123 PRINT *, "Is '123 '"
+!$ABC PRINT *, "Is 'ABC '"
+! $    PRINT *, "This is a comment line 6"
+c    $This is a comment line
+!0$110This is a comment line
+
+! $ This is a comment line
+! $  0This is a comment line
+!    &This is a comment line
+!  $  This is a comment line
+! $   This is a comment line
+C $   This is a comment line
+c $   his is a comment line
+* $   This is a comment line
+      end