Add test (and fix) for a function argument of a macro that expands with a comma.

The fix here is quite simple (and actually only deletes code). When
expanding a macro, we don't return a ',' as a unique token type, but
simply let it fall through to the generic case.

glcpp-parse.y

@@ -779,8 +779,6 @@ glcpp_parser_lex (glcpp_parser_t *parser)
 		return '(';
 	else if (strcmp (replacements->value, ")") == 0)
 		return ')';
-	else if (strcmp (replacements->value, ",") == 0)
-		return ',';
 
 	yylval.str = xtalloc_strdup (parser, replacements->value);
 

tests/039-func-arg-obj-macro-with-comma.c

@@ -0,0 +1,3 @@
+#define foo(a) (a)
+#define bar two,words
+foo(bar)