PR26423: Assert on valid use of using declaration of a function with an undeduced auto return type

For now just disregard the using declaration in this case. Suboptimal,
but wiring up the ability to have declarations of functions that are
separate from their definition (we currently only do that for member
functions) and have differing return types (we don't have any support
for that) is more work than seems reasonable to at least fix this crash.

llvm-svn: 277852