Fix ReferenceStorage(T) atomic if T has no inner pointers

[HertzDevil]

It turns out the fix in #14730 made all ReferenceStorage objects non-atomic; Crystal::ReferenceStorageType#reference_type returns a reference type, whose #has_inner_pointers? always returns true since the reference itself is a pointer:

# CRYSTAL_TRACE=gc crystal run -Dtracing ...

class Foo
end

class Bar
  @x = ""
end

Pointer(ReferenceStorage(Foo)).malloc(1000)
Pointer(ReferenceStorage(Bar)).malloc(1000)

gc.malloc 992475497830875 thread=0x1f716cc00:? fiber=0x1026e6f00:main size=4000 duration=833
gc.malloc 992475497835083 thread=0x1f716cc00:? fiber=0x1026e6f00:main size=16000 duration=2458

This PR fixes that again: (note the presence of atomic=1 on the first line)

gc.malloc 992584569601208 thread=0x1f716cc00:? fiber=0x10259ef00:main size=4000 atomic=1 duration=708
gc.malloc 992584569609041 thread=0x1f716cc00:? fiber=0x10259ef00:main size=16000 duration=6042

[crysbot]

This pull request has been mentioned on Crystal Forum. There might be relevant details there:

https://forum.crystal-lang.org/t/crystal-1-13-2-is-released/7110/1