[clangd] Implementation of textDocument/documentSymbol

Summary:
An AST-based approach is used to retrieve the document symbols rather than an
in-memory index query. The index is not an ideal fit to achieve this because of
the file-centric query being done here whereas the index is suited for
project-wide queries. Document symbols also includes more symbols and need to
keep the order as seen in the file.

Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>

Subscribers: tomgr, ilya-biryukov, ioeric, MaskRay, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D47846

llvm-svn: 336386