[clangd] Disable BackgroundIndexTest.PeriodicalIndex
[clangd] Disable BackgroundIndexTest.PeriodicalIndex

It sometimes fails on AArch64.

git-svn-id: a34e9779ed74578ad5922b3306b3d80a0c825546@350512 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang-tools-extra,llvm-mirror/clang-tools-extra,llvm-mirror/clang-tools-extra,llvm-mirror/clang-tools-extra