[clangd] Support renaming designated initializers

Summary:
Clangd does not find references of designated iniitializers yet and, as a
result, is unable to rename such references. This patch addresses this issue.

Resolves: https://github.com/clangd/clangd/issues/247

Reviewers: sammccall

Reviewed By: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72867