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