[CodeGen][ObjC] Emit the retainRV marker as a module flag instead of

named metadata.

This fixes a bug where ARC contract wasn't inserting the retainRV
marker when LTO was enabled, which caused objects returned from a
function to be auto-released.

rdar://problem/49464214

Differential Revision: https://reviews.llvm.org/D60302

llvm-svn: 358048