Introduce ObjCRuntime::hasWeakClassImport() and use it in the appropriate
places.  I've turned this off for the GNU runtimes --- I don't know if
they support weak class import, but it's easy enough for them to opt in.

Also tweak a comment per review by Jordan.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@158860 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed