rustdoc: Only link to local inlined foreign items

This commit alters rustdoc to keep a hash set of known inlined items which is a
whitelist for generating URLs to.

Closes #14438
auto merge of #14452 : alexcrichton/rust/issue-14438, r=huonw

This commit alters rustdoc to keep a hash set of known inlined items which is a
whitelist for generating URLs to.

Closes #14438