mlir: set CMAKE_INCLUDE_CURRENT_DIR to fix out-of-tree builds

This option tells CMake to add current source and binary
directories to the include path for each directory[1].

Required include directories from build tree (for generated
files) were previously added in `mlir_tablegen` but this was
changed in 03078ec20b12605fd4dfd9fe9c98a26c9d2286d7 .

These are still needed, however, for out-of-tree builds
that don't build as part of LLVM (via LLVM_ENABLE_PROJECTS).
Building as part of LLVM works regardless, AFAICT,
because LLVM sets this option and so the MLIR build inherits it.

FWIW, various other (in-tree) LLVM projects set this as well.

And of course this fixes the out-of-tree
mlir-by-itself build scenario I'm using.

[1] https://cmake.org/cmake/help/latest/variable/CMAKE_INCLUDE_CURRENT_DIR.html

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D122088

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