[clang] [AST] Teach TextNodeDumper to print the "implicit" bit for coroutine AST nodes (PR #77311)

[Ilya Biryukov via cfe-commits]

https://github.com/ilya-biryukov approved this pull request.

LGTM, other nodes also do this.
But please take a look at suggestions for improving the test before landing.

https://github.com/llvm/llvm-project/pull/77311