Since 730e3b2ce0
("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.
Testing this patch on my machine, the fannkuchredux benchmark gets a
1.15 speed-up with YJIT and 1.03 without YJIT.