[flang] Fix missing generated includes in out of tree build

875fd9df added a new dialect
with some generated files.

When flang is built out of tree (build llvm/clang/mlir first, then
build flang pointing at the first build) those files were not created
at all.

I don't 100% understand why not but juding by the comment at the top
of the file, add_mlir_interface probably expects to run in an MLIR
directory, as add_mlir_dialect does.

So in the same way, I've just inlined enough of that function to
fix the out of tree build.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D136250