-Wunreachable-code: refine recognition of unreachable "sigil" to cope with implicit casts in C++.

Fixes <rdar://problem/16631033>.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206360 91177308-0d34-0410-b5e6-96231b3b80d8