[clangd] When replying, log the method name and latency.

Summary:
This information is strictly available in the log (you can find the original
call) but it makes the log easier to follow in practice.

Reviewers: kadircet

Subscribers: ilya-biryukov, ioeric, MaskRay, jkorous, arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D53647

llvm-svn: 345150