[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

.gitignore

@@ -21,6 +21,9 @@
 #OS X specific files.
 .DS_store
 
+# Ignore the user specified CMake presets in subproject directories.
+/*/CMakeUserPresets.json
+
 # Nested build directory
 /build*