When the return type of a function is dependent, don't perform any

of the flow-control checks for falling off the end of a function,
since the return type may instantiate to void. Similarly, if a
return statement has an expression and the return type of the function
is void, don't complain if the expression is type-dependent, since
that type could instantiate to void.

Fixes PR5071.

llvm-svn: 83222