C++1y auto return type: when a function contains no 'return' statements at all,

substitute 'void' into the return type rather than replacing it with 'void', so
that we maintain the 'auto' type sugar.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181584 91177308-0d34-0410-b5e6-96231b3b80d8