[flang] Skip `D` when including D debug line

[jeanPerier]

Summary

When including debug lines as code, the D should be considered as
a white space. Currently an error was raised about bad labels because
it the D remained a D when considering the source line as code.

flang/lib/Parser/prescan.cpp

if (directiveSentinel_[0] == '$' && directiveSentinel_[1] == '\0') {
         ++at_, ++column_;
       }
       tokens.CloseToken();
     }
     break;
   case LineClassification::Kind::Source:
     BeginStatementAndAdvance();
     if (inFixedForm_) {
+      if (features_.IsEnabled(LanguageFeature::OldDebugLines) &&
+          (*at_ == 'D' || *at_ == 'd')) {
+        NextChar();
+      }
       LabelField(tokens);
     } else if (skipLeadingAmpersand_) {
       skipLeadingAmpersand_ = false;
       const char *p{SkipWhiteSpace(at_)};
       if (p < limit_ && *p == '&') {
         column_ += ++p - at_;
         at_ = p;
       }