[SemaObjC] Fix a -Wbridge-cast false-positive

Clang used to emit a bad -Wbridge-cast diagnostic on the cast in the attached
test. This was because, after 09abecef, struct __CFString was not added to
lookup, so the objc_bridge attribute wasn't getting duplicated onto the most
recent declaration, causing us to fail to find it in getObjCBridgeAttr. This
patch fixes this by instead walking through the redeclarations to find an
appropriate bridge attribute. rdar://72823399

Differential revision: https://reviews.llvm.org/D99661