Fix inst_kind for merged interface decl sigs
Fix inst_kind for merged interface decl sigs

Summary:
When converting signatures into types, we use the same `merge_interface`
function for both inline interfaces and interface declarations. This diff fixes
a minor issue where interface declarations had the wrong inst_kind value. The
inst_kind value is used by the type nomalizer to improve printing.

Reviewed By: vrama628

Differential Revision: D23505054

fbshipit-source-id: 9b50e4635731bbd0c26292fd533e49a1c940c46a

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow