[clangd][test] Provide registered targets to lit tests

Summary:
We had tests in clangd (target_info.test) that got enabled only on
systems that know about x86. But they were always disabled as clangd lit config
never registered those targets.

This patch adds those targets as `$TARGET$-registered-target`

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77944