[clangd][vscode] Enable dot-to-arrow fixes in clangd completion.

Summary:
The previous issue is that the item was filtered out by vscode, because
the prefix (which contains ".") are not matched against the filterText.

This patch works around it by adjusting the item filterText, inspired by
https://reviews.llvm.org/D75623.

Reviewers: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75739