Enable out-of-tree projects to use MLIR.

[sscalpone]

Add CMake rules to copy various tools and files into the install directory. Installs the include and td sources, as well as generated inc files, in include/mlir. Installs the binaries mlir-cpu-runner, mlir-cuda-runner, mlir-opt, mlir-translate, and mlir-tblgen in bin.

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