[flang][OpenMP] Fix regression in !$ continuation

A recent patch that obviated the need to use -fopenmp when using
the compiler to preprocess in -E mode broke a case of Fortran line
continuation when using OpenMP conditional compilation lines (!$)
when *not* in -E mode.  Fix.

Author: klausler

flang/lib/Parser/prescan.cpp

@@ -1351,26 +1351,37 @@ const char *Prescanner::FreeFormContinuationLine(bool ampersand) {
   }
   p = SkipWhiteSpaceIncludingEmptyMacros(p);
   if (InCompilerDirective()) {
-    if (preprocessingOnly_ && directiveSentinel_[0] == '$' &&
-        directiveSentinel_[1] == '\0') {
-      // in -E mode, don't treat !$ as a continuation
+    if (directiveSentinel_[0] == '$' && directiveSentinel_[1] == '\0') {
+      if (preprocessingOnly_) {
+        // in -E mode, don't treat !$ as a continuation
+        return nullptr;
+      } else if (p[0] == '!' && p[1] == '$') {
+        // accept but do not require a matching sentinel
+        if (IsLetter(p[2])) {
+          return nullptr; // not !$
+        }
+        p += 2;
+      }
     } else if (*p++ == '!') {
       for (const char *s{directiveSentinel_}; *s != '\0'; ++p, ++s) {
         if (*s != ToLowerCaseLetter(*p)) {
           return nullptr; // not the same directive class
         }
       }
-      p = SkipWhiteSpace(p);
-      if (*p == '&') {
-        if (!ampersand) {
-          insertASpace_ = true;
-        }
-        return p + 1;
-      } else if (ampersand) {
-        return p;
+    } else {
+      return nullptr;
+    }
+    p = SkipWhiteSpace(p);
+    if (*p == '&') {
+      if (!ampersand) {
+        insertASpace_ = true;
       }
+      return p + 1;
+    } else if (ampersand) {
+      return p;
+    } else {
+      return nullptr;
     }
-    return nullptr;
   }
   if (p[0] == '!' && p[1] == '$' && !preprocessingOnly_ &&
       features_.IsEnabled(LanguageFeature::OpenMP)) {