[clang] [clang][ASTImporter] Remove trailing return testing on lambda proto (PR #101031)

[Ding Fei via cfe-commits]

danix800 wrote:

> > Here can `ToProcess.append(CurrentS->child_begin(), CurrentS->child_end());` add any nullptr into `ToProcess` in this testing context?
> 
> I think it is possible, but I could not reproduce this case with the shown code.

We have over 30 C++ open-source projects (including AOSP) in our pipeline to
test against potential crashes in the Static Analyzer with CTU enabled, the pipeline
can surfaces a few problems in the ASTImporter.

I can improve `isAncestorDeclContextOf` if we really can catch crashes.

https://github.com/llvm/llvm-project/pull/101031