Summary:
This CL introduces an integration test directory for MLIR in general, with
vector dialect integration tests in particular as a first working suite. To
run all the integration tests (and currently just the vector suite):

$ cmake --build . --target check-mlir-integration
[0/1] Running the MLIR integration tests
Testing Time: 0.24s
Passed: 22

The general call is to contribute to this integration test directory with more
tests and other suites, running end-to-end examples that may be too heavy for
the regular test directory, but should be tested occasionally to verify the
health of MLIR.

Background discussion at:
https://llvm.discourse.group/t/vectorops-rfc-add-suite-of-integration-tests-for-vector-dialect-operations/1213/

Reviewers: nicolasvasilache, reidtatge, andydavis1, rriddle, ftynse, mehdi_amini, jpienaar, stephenneuendorffer

Reviewed By: nicolasvasilache, stephenneuendorffer

Subscribers: mgorny, mehdi_amini, rriddle, jpienaar, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, liufengdb, stephenneuendorffer, Joonsoo, grosul1, frgossen, Kayjukh, jurahul, msifontes

Tags: #mlir

Differential Revision: https://reviews.llvm.org/D81626