[clangd] Merge results from static/dynamic index.

Summary:
We now hide the static/dynamic split from the code completion, behind a
new implementation of the SymbolIndex interface. This will reduce the
complexity of the sema/index merging that needs to be done by
CodeComplete, at a fairly small cost in flexibility.

Reviewers: hokein

Subscribers: klimek, mgorny, ilya-biryukov, cfe-commits

Differential Revision: https://reviews.llvm.org/D42049

llvm-svn: 322480