[clangd] Make output order of allTargetDecls deterministic

Summary:
Makes use of insertion order to stabilize output for multiple decls.

Fixes https://bugs.llvm.org/show_bug.cgi?id=44564

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72826

(cherry picked from commit d54d71b6)