[flang] [mlir rebase] Add MLIR config and react to MLIR name changes (flang-compiler/f18#1090)

[mlir rebase] Add MLIR config and react to MLIR name changes

Similar to flang-compiler/f18#1085.  Now use the MLIR package to set up paths for include files and libraries.  Three MLIR names changed:
 * VectorOpsDialect to VectorDialect
 * AffineOpsDialect to AffineDialect
 * createVectorizePass to createSuperVectorizePass

Update README.md to explain how to link with MLIR. Update the example gcc to version 8.3.

Update drone.io config to define -DMLIR_DIR

Co-authored-by: Jean Perier <jperier@nvidia.com>

Original-commit: flang-compiler/f18@116f64315f6218a7f74a8ccdc25f94d322ac6344
Reviewed-on: https://github.com/flang-compiler/f18/pull/1090