Auto merge of #32667 - dotdash:def_dec, r=nagisa
Auto merge of #32667 - dotdash:def_dec, r=nagisa

Stop accepting declarations when looking for definitions

eternally_available linkage isn't legal for declarations anyway, so the
check for an externally_available declaration should never succeed, so
let's remove it.

graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust