[clang-format] Fix cleanup of `AnnotatedLine` to include children nodes.

Summary:
AnnotatedLine has a tree structure, and things like the body of a lambda will be
a child of the lambda expression. For example,

    [&]() { foo(a); };

will have an AnnotatedLine with a child:

    [&]() {};
     '- foo(a);

Currently, when the `Cleaner` class analyzes the affected lines, it does not
cleanup the lines' children nodes, which results in missed cleanup
opportunities, like the lambda body in the example above.

This revision extends the algorithm to visit children, thereby fixing the above problem.

Patch by Eric Li.

Reviewers: krasimir

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D67659

llvm-svn: 372129