[OpenACC] Handle lack of construct/directive

Discovered while working on another patch, this patch fixes the case
where are construct/directive name isn't provided.

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -1357,6 +1357,7 @@ def warn_pragma_acc_unimplemented_clause_parsing
       InGroup<SourceUsesOpenACC>;
 def err_acc_invalid_directive
     : Error<"invalid OpenACC directive '%0'">;
+def err_acc_missing_directive : Error<"expected OpenACC directive">;
 
 // OpenMP support.
 def warn_pragma_omp_ignored : Warning<

clang/lib/Parse/ParseOpenACC.cpp

@@ -42,6 +42,14 @@ OpenACCDirectiveKind GetOpenACCDirectiveKind(StringRef Name) {
 // Parse and consume the tokens for OpenACC Directive/Construct kinds.
 OpenACCDirectiveKind ParseOpenACCDirectiveKind(Parser &P) {
   Token FirstTok = P.getCurToken();
+
+  // Just #pragma acc can get us immediately to the end, make sure we don't
+  // introspect on the spelling before then.
+  if (FirstTok.isAnnotation()) {
+    P.Diag(FirstTok, diag::err_acc_missing_directive);
+    return OpenACCDirectiveKind::Invalid;
+  }
+
   P.ConsumeToken();
   std::string FirstTokSpelling = P.getPreprocessor().getSpelling(FirstTok);
 

clang/test/ParserOpenACC/parse-constructs.c

@@ -1,6 +1,12 @@
 // RUN: %clang_cc1 %s -verify -fopenacc
 
 void func() {
+
+  // expected-error@+2{{expected OpenACC directive}}
+  // expected-warning@+1{{OpenACC directives not yet implemented, pragma ignored}}
+#pragma acc
+  for(;;){}
+
   // expected-error@+2{{invalid OpenACC directive 'invalid'}}
   // expected-warning@+1{{OpenACC directives not yet implemented, pragma ignored}}
 #pragma acc invalid