[NFC] Add CMakeUserPresets.json filename to .gitignore

CMake 3.19 introduced the `presets`.
Quoting the documentation:
> `CMakePresets.json` may be checked into a version control system, and
> `CMakeUserPresets.json` **should NOT be checked in**.

We will ignore the `CMakeUserPresets.json` file if that is present
at the root of a subproject.

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D93167