re-enable -Wunreachable-code under clang

whitelist the valid cases showing up

If an earlier version of clang is showing more cases (particularly
if they don't make sense), the re-enabling of the warning in
sys/unix/hints/include/compiler.2020 can be made clang-version
specific instead. I had no way to test earlier versions.

src/nhlua.c

@@ -116,6 +116,8 @@ l_nhcore_call(int callidx)
     }
 }
 
+DISABLE_WARNING_UNREACHABLE_CODE
+
 void
 nhl_error(lua_State *L, const char *msg)
 {
@@ -136,8 +138,11 @@ nhl_error(lua_State *L, const char *msg)
 #endif
     (void) lua_error(L);
     /*NOTREACHED*/
+    /* UNREACHABLE_CODE */
 }
 
+RESTORE_WARNING_UNREACHABLE_CODE
+
 /* Check that parameters are nothing but single table,
    or if no parameters given, put empty table there */
 void