DebugInfo: Remove auto return type representation support

Seems this complicated lldb sufficiently for some cases that it hasn't
been worth supporting/fixing there - and it so far hasn't provided any
new use cases/value for debug info consumers, so let's remove it until
someone has a use case for it.

(side note: the original implementation of this still had a bug (I
should've caught it in review) that we still didn't produce
auto-returning function declarations in types where the function wasn't
instantiatied (that requires a fix to remove the `if
getContainedAutoType` condition in
`CGDebugInfo::CollectCXXMemberFunctions` - without that, auto returning
functions were still being handled the same as member function templates
and special member functions - never added to the member list, only
attached to the type via the declaration chain from the definition)

Further discussion about this in D123319

This reverts commit 5ff992bc: [DEBUG-INFO] Change how we handle auto return types for lambda operator() to be consistent with gcc

This reverts commit c83602fd: [DWARF5][clang]: Added support for DebugInfo generation for auto return type for C++ member functions.

Differential Revision: https://reviews.llvm.org/D131933