[Coroutine] Add statistics for the number of elided coroutine

Now we lack a benchmark to measure the performance change for each
commit.
Since coro elide is the main optimization in coroutine module, I wonder
it may be an estimation to count the number of elided coroutine in
private code bases.
e.g., for a certain commit, if we found that the number of elided goes
down, we could find it before the commit check-in.

Reviewed By: lxfind

Differential Revision: https://reviews.llvm.org/D105095