[clangd] Refine logic on $0 in completion snippets

We have a workaround from D128621 that makes $0 no longer being
a placeholder to conform a vscode feature. However, we have to
refine the logic as it may suppress the last parameter placeholder
for constructor of base class because not all patterns of completion
are compound statements.

This fixes clangd/clangd#1479

Reviewed By: nridge

Differential Revision: https://reviews.llvm.org/D145319