[clangd] Don't create child AND and OR iterators with one posting list

`AND( AND( Child ) ... )` -> `AND( Child ... )`
`AND( OR( Child ) ... )` -> `AND( Child ... )`

This simple optimization results in 5-6% performance improvement in the
benchmark with 2000 serialized `FuzzyFindRequest`s.

Reviewed By: ilya-biryukov

Differential Revision: https://reviews.llvm.org/D52016

llvm-svn: 342124