Support implicit _Atomic struct load / store
Summary:
Using _Atomic to do implicit load / store is just a seq_cst atomic_load / atomic_store. Stores currently assert in Sema::ImpCastExprToType with 'can't implicitly cast lvalue to rvalue with this cast kind', but that's erroneous. The codegen is fine as the test shows.
While investigating I found that Richard had found the problem here: https://reviews.llvm.org/D46112#1113557
<rdar://problem/
40347123>
Reviewers: dexonsmith
Subscribers: cfe-commits, efriedma, rsmith, aaron.ballman
Differential Revision: https://reviews.llvm.org/D49458
llvm-svn: 337410