Mark rb_clear_constant_cache as internal use only

In the past, many internal functions are declared in intern.h
under include/ruby directory, because there were no headers for
internal use.

include/ruby/backward.h

@@ -13,6 +13,9 @@
 
 #define RBIMPL_ATTR_DEPRECATED_SINCE(ver) RBIMPL_ATTR_DEPRECATED(("since " #ver))
 #define RBIMPL_ATTR_DEPRECATED_INTERNAL(ver) RBIMPL_ATTR_DEPRECATED(("since "#ver", also internal"))
+#define RBIMPL_ATTR_DEPRECATED_INTERNAL_ONLY() RBIMPL_ATTR_DEPRECATED(("only for internal use"))
+
+RBIMPL_ATTR_DEPRECATED_INTERNAL_ONLY() void rb_clear_constant_cache(void);
 
 /* from version.c */
 #if defined(RUBY_SHOW_COPYRIGHT_TO_DIE) && !!(RUBY_SHOW_COPYRIGHT_TO_DIE+0)

include/ruby/internal/intern/vm.h

@@ -252,17 +252,6 @@ void rb_undef_alloc_func(VALUE klass);
  */
 rb_alloc_func_t rb_get_alloc_func(VALUE klass);
 
-/**
- * Clears  the constant  cache.   Extension libraries  should  not bother  such
- * things.   Just forget  about this  API (or  even, the  presence of  constant
- * cache).
- *
- * @internal
- *
- * Completely no idea why this function is defined in vm_method.c.
- */
-void rb_clear_constant_cache(void);
-
 /**
  * Resembles `alias`.
  *

internal/vm.h

@@ -96,6 +96,8 @@ MJIT_SYMBOL_EXPORT_BEGIN
 int rb_ec_obj_respond_to(struct rb_execution_context_struct *ec, VALUE obj, ID id, int priv);
 MJIT_SYMBOL_EXPORT_END
 
+void rb_clear_constant_cache(void);
+
 /* vm_dump.c */
 void rb_print_backtrace(void);