Syntax tree: ignore implicit expressions at the top level of statements

Summary:
I changed `markStmtChild` to ignore implicit expressions the same way as
`markExprChild` does it already. The test that I modified crashes
without this change.

Reviewers: hlopko, eduucaldas

Reviewed By: hlopko, eduucaldas

Subscribers: gribozavr2, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81019