@c @implementation relies on matching the original C declaration. The
lookup for the original C declaration was doing the wrong kind of
lookup, meaning that it could only find the C declaration if it came
through a bridging header, and not through a normal module import.
Using unqualified lookup here finds the name appropriately.
Clarify the diagnostics here as well to not talk about umbrella and
bridging headers.
Fixes rdar://161909754.
