[ObjC] Supress the 'implementing unavailable method' warning when

[ObjC] Supress the 'implementing unavailable method' warning when
the method declaration is unavailable for an app extension platform

Rationale:
Classes are often shared between an app extension code and
non-app extension code. There's no way to remove the implementation
using preprocessor when building the app extension, so we should not warn here.

rdar://38150617

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@331421 91177308-0d34-0410-b5e6-96231b3b80d8