refactor: use CLEAR_FIELD and CLEAR_POINTER macros (#19709)

vim-patch:8.2.0559: clearing a struct is verbose

Problem:    Clearing a struct is verbose.
Solution:   Define and use CLEAR_FIELD() and CLEAR_POINTER().
a80faa8930

src/nvim/regexp_bt.c

@@ -466,7 +466,7 @@ static void regcomp_start(char_u *expr, int re_flags)                        //
 
   num_complex_braces = 0;
   regnpar = 1;
-  memset(had_endbrace, 0, sizeof(had_endbrace));
+  CLEAR_FIELD(had_endbrace);
   regnzpar = 1;
   re_has_z = 0;
   regsize = 0L;