[MLIR] [CMake] Support building MLIR standalone

Add the necessary bits to CMakeLists to make it possible to configure
MLIR against installed LLVM, and build it with minimal need for LLVM
source tree.  The latter is only necessary to run unittests, and if it
is missing then unittests are skipped with a warning.

This change includes the necessary changes to tests, in particular
adding some missing substitutions and defining missing variables
for lit.site.cfg.py substitution.

Reviewed By: stephenneuendorffer

Differential Revision: https://reviews.llvm.org/D85464



Co-authored-by: Isuru Fernando <isuruf@gmail.com>