[clangd] Modify the Span API so that Spans propagate with contexts.

Summary:
This is probably the right behavior for distributed tracers, and makes unpaired
begin-end events impossible without requiring Spans to be bound to a thread.

The API is conceptually clean but syntactically awkward. As discussed offline,
this is basically a naming problem and will go away if (when) we use TLS to
store the current context.

The apparently-unrelated change to onScopeExit are because its move semantics
broken if Func is POD-like since r322838. This is true of function pointers,
and the lambda I use here that captures two pointers only.
I've raised this issue on llvm-dev and will revert this part if we fix it in
some other way.

Reviewers: ilya-biryukov

Subscribers: klimek, jkorous-apple, ioeric, cfe-commits

Differential Revision: https://reviews.llvm.org/D42499

llvm-svn: 323511