[clangd][Hover] Handle uninstantiated default args

Summary:
Default args might exist but be unparsed or uninstantiated.
getDefaultArg asserts on those. This patch makes sure we don't crash in such
scenarios.

Reviewers: sammccall, ilya-biryukov

Subscribers: MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73723

(cherry-picked from commit 9c903d03)

Fixes https://github.com/clangd/clangd/issues/424