[clangd] Add textDocument/signatureHelp

Summary:
Makes clangd respond to a client's "textDocument/signatureHelp" request by
presenting function/method overloads.

Patch by Raoul Wols.

Reviewers: bkramer, ilya-biryukov, krasimir

Reviewed By: ilya-biryukov

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D38048

llvm-svn: 315055