form_match_list(): Don't use _Atomic

There is no need to ensure the "id" variable is incremented in an atomic
way, because we don't care if two threads will use the same value.

The crash that got debugged on commit 9f73b37 was due to another
reason: not using "id" variable if it is 0. This has been fixed.

Removing it gets rid of some small overhead, and also allows MSVC compilation.

link-grammar/parse/fast-match.c

@@ -595,8 +595,8 @@ form_match_list(fast_matcher_t *ctxt, int w,
 	}
 
 #ifdef VERIFY_MATCH_LIST
-	static _Atomic(uint16_t) id = 0;
-	uint16_t lid = ++id; /* A local copy, for multi-threading support. */
+	static uint16_t id = 0;
+	uint16_t lid = ++id; /* A stable local copy, for multi-threading support. */
 #else
 	const uint16_t lid = 0;
 #endif