[clangd] Implement textDocument/foldingRange

Summary:
This patch introduces basic textDocument/foldingRange support. It relies on
textDocument/documentSymbols to collect all symbols and uses takes ranges
to create folds.

The next steps for textDocument/foldingRange support would be:

* Implementing FoldingRangeClientCapabilities and respecting respect client
  preferences
* Specifying folding range kind
* Migrating from DocumentSymbol implementation to custom RecursiveASTVisitor flow that will allow more flexibility
* Supporting more folding range types: comments, PP conditional regions, includes and other code regions (e.g. public/private/protected sections of classes, control flow statement bodies)

Tested: (Neo)Vim (coc-clangd) and VSCode.

Related issue: https://github.com/clangd/clangd/issues/310

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: nridge, ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D82436