[clangd] Take into account what is in the index (symbols, references, etc.) at indexes merge

Current indexes merge logic skip data from the static index if the file is in the dynamic index, but sometimes the dynamic index does not contain references (e.g. preamble (dynamic) index vs background (static) index).
This problem is masked with the fact, that the preamble index file list consists of file URI's and other indexes file lists consist of file paths.
This patch introduces the index contents (symbols, references, etc.), which makes indexes merge more flexible and makes it able to use URI's for the index file list.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D94952