PR18746: If a constexpr function has a dependent return type and no return
statements, don't diagnose; the return type might end up being 'void'.

Patch by Rahul Jain! Tiny tweaks by me.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206929 91177308-0d34-0410-b5e6-96231b3b80d8