[clangd] Changed tracing interfaces

Summary:
EventTracer interface now contains two methods:
- spanEvent for events that have duration,
- instant for events that are instant.

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: klimek, luckygeck, cfe-commits

Differential Revision: https://reviews.llvm.org/D40489

llvm-svn: 320708