[Support] Add support for attaching payloads to points and ranges

This is useful where tests previously encoded information in the name
names of ranges and points. Currently, this is pretty limited because
names consist of only alphanumeric characters and '_'.

With this patch, we can keep the names simple and attach optional
payloads to ranges and points instead.

The new syntax should be fully backwards compatible (if I haven't missed
anything). I tested this against clangd unit tests and everything still passes.

Differential Revision: https://reviews.llvm.org/D137909