[clangd] Report position of opening paren in singature help

Summary: Only accessible via the C++ API at the moment.

Reviewers: sammccall

Reviewed By: sammccall

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

Differential Revision: https://reviews.llvm.org/D51437

llvm-svn: 341065