[flang][openacc] Suppport !@acc compiler sentinel

flang/lib/Parser/parsing.cpp

@@ -85,6 +85,7 @@ const SourceFile *Parsing::Prescan(const std::string &path, Options options) {
   if (options.features.IsEnabled(LanguageFeature::OpenACC) ||
       (options.prescanAndReformat && noneOfTheAbove)) {
     prescanner.AddCompilerDirectiveSentinel("$acc");
+    prescanner.AddCompilerDirectiveSentinel("@acc");
   }
   if (options.features.IsEnabled(LanguageFeature::OpenMP) ||
       (options.prescanAndReformat && noneOfTheAbove)) {

flang/lib/Parser/prescan.cpp

@@ -147,6 +147,11 @@ void Prescanner::Statement() {
         directiveSentinel_[4] == '\0') {
       // CUDA conditional compilation line.
       condOffset = 5;
+    } else if (directiveSentinel_[0] == '@' && directiveSentinel_[1] == 'a' &&
+        directiveSentinel_[2] == 'c' && directiveSentinel_[3] == 'c' &&
+        directiveSentinel_[4] == '\0') {
+      // OpenACC conditional compilation line.
+      condOffset = 5;
     }
     if (condOffset && !preprocessingOnly_) {
       at_ += *condOffset, column_ += *condOffset;

flang/test/Semantics/OpenACC/acc-sentinel.f90

@@ -0,0 +1,14 @@
+! RUN: %python %S/../test_errors.py %s %flang_fc1 -fopenacc
+
+subroutine test_sentinel()
+! Test for error since we currently do not have an OpenACC module upstream.
+!ERROR: Cannot parse module file for module 'openacc': Source file 'openacc.mod' was not found
+  !@acc use openacc
+  integer :: i
+
+  !$acc parallel loop
+  do i = 1, 10
+  end do
+  !$acc end parallel
+
+end subroutine