[libc++] Add empty release notes file for LLVM 20 (#98355)

While we haven't branched yet, this will make it easier for folks to add
release notes for PRs that won't be merged before the LLVM 19 branch
point.

NOKEYCHECK=True
GitOrigin-RevId: 33af4bd7a4c42f55646fa7d2826cd41384ecdece
2 files changed