PR16875: The return type of a dependent function type is visible when it's

referenced as a member of the current instantiation. In that case, deduce the
type of the function to a dependent type rather than exposing an undeduced auto
type to the rest of the current instantiation.

The standard doesn't really say that the type is dependent in this case; I'll
bring this up with CWG.

llvm-svn: 188410