[clangd] Don't produce snippets when completion location is followed by parenthesis

Summary:
Prevent a second pair of parenthesis from being added when there already is one
right after cursor.

Related issue and more context: https://github.com/clangd/clangd/issues/387

Reviewers: sammccall

Reviewed By: sammccall

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81380