[clangd] Merge ClangdServer::DynamicIndex into FileIndex. NFC.

Summary:
FileIndex now provides explicit interfaces for preamble and main file updates.
This avoids growing parameter list when preamble and main symbols diverge
further (e.g. D52078). This also gets rid of the hack in `indexAST` that
inferred main file index based on `TopLevelDecls`.

Also separate `indexMainDecls` from `indexAST`.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D52222

llvm-svn: 342460