ObjectiveC. Further improvements of use

ObjectiveC. Further improvements of use
of objc_bridge_related attribute; eliminate
unnecessary diagnostics which is issued elsewhere,
fixit now produces a valid AST tree per convention.
This results in some simplification in handling of
this attribute as well. // rdar://15499111

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@197436 91177308-0d34-0410-b5e6-96231b3b80d8