[flang][openmp] Handle !$INCLUDE "foo" (bug #64128)

[klausler]

Summary

Detect and process INCLUDE lines that are guarded by OpenMP conditional
compilation markers (!$), when enabled.

Fixes https://github.com/llvm/llvm-project/issues/64128.

[kiranchandramohan]

LG. Thanks.

flang/lib/Parser/prescan.cpp

case LineClassification::Kind::IncludeLine:
     NextLine();
     return;
   case LineClassification::Kind::ConditionalCompilationDirective:
   case LineClassification::Kind::IncludeDirective:
   case LineClassification::Kind::DefinitionDirective:
   case LineClassification::Kind::PreprocessorDirective:
     preprocessor_.Directive(TokenizePreprocessorDirective(), *this);
     return;
-  case LineClassification::Kind::CompilerDirective:
+  case LineClassification::Kind::CompilerDirective: {
     directiveSentinel_ = line.sentinel;
     CHECK(InCompilerDirective());
     BeginStatementAndAdvance();
     if (inFixedForm_) {
       CHECK(IsFixedFormCommentChar(*at_));
     } else {
       while (*at_ == ' ' || *at_ == '\t') {
         ++at_, ++column_;
       }
       CHECK(*at_ == '!');
     }
+    std::optional<int> condOffset;
     if (directiveSentinel_[0] == '$' && directiveSentinel_[1] == '\0') {
-      // OpenMP conditional compilation line.  Remove the sentinel and then
-      // treat the line as if it were normal source.
-      at_ += 2, column_ += 2;
-      if (inFixedForm_) {
-        LabelField(tokens);
-      } else {
-        SkipSpaces();
-      }
+      // OpenMP conditional compilation line.
+      condOffset = 2;
     } else if (directiveSentinel_[0] == '@' && directiveSentinel_[1] == 'c' &&
         directiveSentinel_[2] == 'u' && directiveSentinel_[3] == 'f' &&
         directiveSentinel_[4] == '\0') {
-      // CUDA conditional compilation line.  Remove the sentinel and then
-      // treat the line as if it were normal source.
-      at_ += 5, column_ += 5;
-      if (inFixedForm_) {
+      // CUDA conditional compilation line.
+      condOffset = 5;
+    }
+    if (condOffset) {
+      at_ += *condOffset, column_ += *condOffset;
+      if (auto payload{IsIncludeLine(at_)}) {
+        FortranInclude(at_ + *payload);
+        return;
+      } else if (inFixedForm_) {
         LabelField(tokens);
       } else {
         SkipSpaces();
       }
     } else {
       // Compiler directive.  Emit normalized sentinel.
       EmitChar(tokens, '!');
       ++at_, ++column_;
       for (const char *sp{directiveSentinel_}; *sp != '\0';
            ++sp, ++at_, ++column_) {
         EmitChar(tokens, *sp);
       }
       if (*at_ == ' ') {
         EmitChar(tokens, ' ');
         ++at_, ++column_;
       }
       tokens.CloseToken();
     }
     break;
+  }
   case LineClassification::Kind::Source:
     BeginStatementAndAdvance();
     if (inFixedForm_) {
       if (features_.IsEnabled(LanguageFeature::OldDebugLines) &&
           (*at_ == 'D' || *at_ == 'd')) {
         NextChar();
       }
       LabelField(tokens);

flang/test/Parser/OpenMP/cond-include.f90

+! RUN: %flang_fc1 -fdebug-unparse -fopenmp %s | FileCheck %s
+!CHECK: STOP "pass"
+!$ include "cond-include.inc"
+end

flang/test/Parser/OpenMP/cond-include.inc

+STOP "pass"