[OpenACC] Implement 'nohost' construct AST/Sema

'nohost' is only valid on routine, and states that the compiler
shouldn't compile this routine for the host. It has no arguments, so no
checking is required besides putting it in the AST.

clang/lib/Serialization/ASTWriter.cpp

@@ -8783,6 +8783,7 @@ void ASTRecordWriter::writeOpenACCClause(const OpenACCClause *C) {
   }
   case OpenACCClauseKind::Seq:
   case OpenACCClauseKind::Independent:
+  case OpenACCClauseKind::NoHost:
   case OpenACCClauseKind::Auto:
   case OpenACCClauseKind::Finalize:
   case OpenACCClauseKind::IfPresent:
@@ -8843,7 +8844,6 @@ void ASTRecordWriter::writeOpenACCClause(const OpenACCClause *C) {
     return;
   }
 
-  case OpenACCClauseKind::NoHost:
   case OpenACCClauseKind::Bind:
   case OpenACCClauseKind::Invalid:
     llvm_unreachable("Clause serialization not yet implemented");