Prep for new warnings about control flow falling off the ends of

Prep for new warnings about control flow falling off the ends of
functions that return a value.  I was going to buffer the whole lot
up, but it should be easier to review if I check them in
incrementally.  Most of the forth coming changes either add a return
value, or make it impossible to return, or alter the return type.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@76605 91177308-0d34-0410-b5e6-96231b3b80d8