rollup merge of #19457: reem/remove-is-lang-item

Removes a FIXME on closed issue #15064. This flag is no
longer needed or used since reflection is gone.

diff --cc src/librustc_trans/trans/base.rs
Simple merge

diff --cc src/librustc_trans/trans/callee.rs
Simple merge

diff --cc src/librustc_trans/trans/glue.rs
Simple merge