Fix stack overflow in linkage computation when a function with a deduced return

type returns a lambda defined within itself. The computation of linkage for the
function looked at the linkage of the lambda, and vice versa.

This is solved by not checking whether an 'auto' in a function return type
deduces to a type with unique external linkage. We don't need this check,
because the type deduced for 'auto' doesn't affect whether two
otherwise-identical declarations would name different functions, so we don't
need to give an ostensibly external-linkage function internal linkage for this
reason. (We also don't need unique-external linkage in C++11 onwards at all,
but that's not implemented yet.)

llvm-svn: 181675