grammar/OmpLexer.g: defines OpenMP keywords
grammar/OmpParser.g: defines OpenMP grammar rules, uses OmpLexer, 
    imports CivlCParser to reuse the rule for expressions
antlr2ast.common.OMPPragmaHandler: translating an OpenMP pragma 
    into OpenMP AST nodes. Note: the source is first parsed by the
    general parser (CivlCParser), and then at the AST builder phase,
    the OMPPragmaHandler is triggered to translate the OpenMP pragma.
    The input is a general pragma node produced by applying the general parser
    (CivlCParser) to the pragma source. 
    
- add a new keyword, for example, "barrier"
  (1) add a token for the keyword in OmpLexer.g: BARRIER: 'barrier';
  (2) add a case for the keyword in OMPPragmaHandler.markTokens():
          case "barrier":
			token.setType(BARRIER);
		  break;
      Since the OpenMP parser uses extra tokens provided by OmpLexer, 
      it needs to mark those tokens explicitly, which are all
      considered as "IDENTIFIER" by CivlCParser. 
      
- add a new BNF rule
  (1) add the rule in OmpParser.g
  (2) add a method for translating the rule in OMPPragmaHandler