Make rb_check_frozen_inline() static inline again

Since 730e3b2
("Stop exposing `rb_str_chilled_p`"), we noticed a speed loss on a few
benchmarks that are string operations-heavy. This is partially due to
routines no longer having the options to inline rb_check_frozen_inline()
in non-LTO builds. Make it an inlining candidate again to recover speed.

On the fannkuchredux benchmark, this patch gives a 1.15 speed-up with
YJIT and 1.03 without YJIT.

error.c

@@ -3997,19 +3997,6 @@ rb_error_frozen_object(VALUE frozen_obj)
     rb_exc_raise(exc);
 }
 
-#undef rb_check_frozen
-void
-rb_check_frozen(VALUE obj)
-{
-    if (RB_UNLIKELY(RB_OBJ_FROZEN(obj))) {
-        rb_error_frozen_object(obj);
-    }
-
-    if (RB_UNLIKELY(CHILLED_STRING_P(obj))) {
-        rb_str_modify(obj);
-    }
-}
-
 void
 rb_check_copyable(VALUE obj, VALUE orig)
 {

include/ruby/internal/intern/error.h

@@ -208,7 +208,7 @@ void rb_error_frozen_object(VALUE what);
  * @exception  rb_eFrozenError  It is frozen.
  * @post       Upon successful return it is guaranteed _not_ frozen.
  */
-void rb_check_frozen(VALUE obj);
+#define rb_check_frozen rb_check_frozen_inline
 
 /**
  * Ensures that the passed object  can be `initialize_copy` relationship.  When
@@ -237,6 +237,9 @@ RBIMPL_ATTR_NORETURN()
  */
 void rb_error_arity(int argc, int min, int max);
 
+bool rb_str_chilled_p(VALUE str);
+void rb_str_modify(VALUE str);
+
 RBIMPL_SYMBOL_EXPORT_END()
 
 /**
@@ -247,7 +250,19 @@ RBIMPL_SYMBOL_EXPORT_END()
 #define rb_check_frozen_internal rb_check_frozen
 
 /** @alias{rb_check_frozen} */
-#define rb_check_frozen_inline rb_check_frozen
+static inline void
+rb_check_frozen_inline(VALUE obj)
+{
+    if (RB_UNLIKELY(RB_OBJ_FROZEN(obj))) {
+        rb_error_frozen_object(obj);
+    }
+
+    // ref: internal CHILLED_STRING_P()
+    // This is implementation details subject to change.
+    if (RB_UNLIKELY(RB_TYPE_P(obj, T_STRING) && FL_TEST_RAW(obj, RUBY_FL_USER3))) {
+        rb_str_modify(obj);
+    }
+}
 
 /**
  * Ensures that the  passed integer is in  the passed range.  When  you can use