[Cherry-pick into next] [lldb] Add missing return type annotations

[adrian-prantl]

commit 2e2508c4df4f89ae1583c91206666d465f7de2e6
Author: Adrian Prantl <[email protected]>
Date:   Fri Jun 13 08:39:34 2025 -0700

    [lldb] Add missing return type annotations
    
    In a thorny corner of the language C++ doesn't actually type check the
    return type of a function passed into a std::function argument. Here
    the type inference in the lambda resulted in a ChildInfo being
    returned where an llvm::Expected<ChildInfo> was expected, which
    through coincidence works very often, but not always, leading to a
    crash.
    
    rdar://153137542