Set the objc constraint on the context based on the parsed files.

Like arch, os, etc, when we know we are going to use a file, we check
that the file has compatible objc constraints to the context, throw
appropriate errors where that is not the case, and hopefully set the
objc constraints on the context for use later.

Added 2 tests to ensure that we don't have incompatibilities between
host and simulator code as both will get x86 based architectures.

llvm-svn: 258173