* Update LLVM commit ID to include to the new modeling of LLVM type in MLIR
* Fix commit id discrepancy
* Update README.md
* Update MLIR version
* Force rebuild prereq dockers and see what happens.
* Use LLVM commit ID that corresponds to MLIR News, 13th edition (8/7/2020)
Co-authored-by: Tian Jin <tjingrant@gmail.com>