bazel build --incompatible_no_implicit_file_export

The Bazel build was relying, for the two files enumerated in this diff, on the legacy implicit-export semantics described here:
https://bazel.build/reference/be/functions#exports_files

This documentation page encourages migrating away from this legacy behavior, and indeed we have a user who reported a Bazel build error and it appears that they were already using the new, stricter behavior:
https://github.com/openxla/iree/pull/13982
and while examining fixes on our side and trying to get a clean Bazel build, I ran into this similar issue in the LLVM overlay.

It would arguably be cleaner (in the sense of more structured) to rely on `filegroup` to export this, but I am insufficiently familiar with the Clang build (the dependent targets seem to be below Clang) to do this myself. The present `exports_files` solution has the merit of being localized in these few lines here.

Differential Revision: https://reviews.llvm.org/D152491