[-Wunreachable-code] generalize pruning out warning on trivial returns.

Previously we only pruned dead returns preceded by a call to a
'noreturn' function.  After looking at the results of the LLVM codebase,
there are many others that should be pruned as well.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203029 91177308-0d34-0410-b5e6-96231b3b80d8