[Coroutines] Fix code coverage for coroutine

Summary:
Previously, source-based coverage analysis does not work properly for coroutine.
This patch adds processing of coroutine body and co_return in the coverage analysis, so that we can handle them properly.
For coroutine body, we should only look at the actual function body and ignore the compiler-generated things; for co_return, we need to terminate the region similar to return statement.
Added a test, and confirms that it now works properly. (without this patch, the statement after the if statement will be treated wrongly)

Reviewers: lewissbaker, modocache, junparser

Reviewed By: modocache

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D82928