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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188410 91177308-0d34-0410-b5e6-96231b3b80d8