Parser: Fix spans of explicit self arg idents
Adjust orphan rules to consider all input types, not just self type.

Fixes #18222.
auto merge of #18141 : phildawes/rust/master, r=brson

Hello! I noticed spans are wrong for the PatIdents of self args. (I use spans a lot in racer)
auto merge of #18223 : nikomatsakis/rust/coherence-orphan-18222, r=pcwalton

Adjust orphan rules to consider all input types, not just self type.

Fixes #18222.

r? @pcwalton
Merge remote-tracking branch 'orig/master' into ios