[clangd] Check preceding char when completion triggers on ':' or '>'

Summary:
Only run completion when we were trigerred on '->' and '::', otherwise
send an error code in return.
To avoid automatically invoking completions in cases like 'a >^' or
'a ? b :^'.

Reviewers: hokein

Reviewed By: hokein

Subscribers: ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D55994

llvm-svn: 350304