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

[Balázs Kéri via cfe-commits]

balazske wrote:

My concern was only that the return type check check can be too much overhead now if done for all C++11 lambdas but probably this type of lambda can not be recognized in other way (maybe from source locations?). But it is only the C++11 (not newer) case and it is more safe to check the type all times so I can accept it.

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