* [MLIR] Bump MLIR repo 8/20/2019

MLIR_
commit 0cdb20a6add19bc96c20dad28589a1e54e4d8469
Author: Lei Zhang <antiagainst@google.com>
Date:   Tue Aug 20 13:33:41 2019 -0700

    Add spv.specConstant and spv._reference_of

LLVM:
commit 3b9a27b6908040881dad394022f8c472c15c0784
Author: Simon Pilgrim <llvm-dev@redking.me.uk>
Date:   Tue Aug 20 17:54:37 2019 +0000

    Fix typo in comment. NFCI.

* Address Bob's feedback