[DEBUG-INFO] Change how we handle auto return types for lambda operator() to be consistent with gcc

D70524 added support for auto return types for C++ member functions. I was
implementing support on the LLDB side for looking up the deduced type.

I ran into trouble with some cases with respect to lambdas. I looked into
how gcc was handling these cases and it appears gcc emits the deduced return type for lambdas.

So I am changing out behavior to match that.

Differential Revision: https://reviews.llvm.org/D123319