refactor: add pure attribute to pure functions (#18165)

This will allow the compilers that support the pure attribute to make
further optimizations.

src/nvim/ex_cmds2.c

@@ -201,6 +201,7 @@ static char *pexpand_cmds[] = {
 /// Function given to ExpandGeneric() to obtain the profile command
 /// specific expansion.
 char_u *get_profile_name(expand_T *xp, int idx)
+  FUNC_ATTR_PURE
 {
   switch (pexpand_what) {
   case PEXP_SUBCMD:
@@ -439,6 +440,7 @@ static void script_dump_profile(FILE *fd)
 /// @return  true when a function defined in the current script should be
 ///          profiled.
 bool prof_def_func(void)
+  FUNC_ATTR_PURE
 {
   if (current_sctx.sc_sid > 0) {
     return SCRIPT_ITEM(current_sctx.sc_sid).sn_pr_force;
@@ -1732,6 +1734,7 @@ int *source_dbg_tick(void *cookie)
 
 /// @return  the nesting level for a source cookie.
 int source_level(void *cookie)
+  FUNC_ATTR_PURE
 {
   return ((struct source_cookie *)cookie)->level;
 }
@@ -2288,6 +2291,7 @@ void free_scriptnames(void)
 #endif
 
 linenr_T get_sourced_lnum(LineGetter fgetline, void *cookie)
+  FUNC_ATTR_PURE
 {
   return fgetline == getsourceline
         ? ((struct source_cookie *)cookie)->sourcing_lnum