[clangd] Add "Go to Declaration" functionality

Summary: This change allows to navigate to most identifiers' declarations in code. This is a first step towards implementing "Go to Definition". It reuses clangIndex in order to detect which occurrences corresponds to the position requested. The occurrences' Decls are then used to generate locations suitable for navigating to the declarations.

Reviewers: krasimir, bkramer, ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: cfe-commits, mgorny

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D34269

llvm-svn: 306558