[-Wunreachable-code] Handle idiomatic do...while() with an uninteresting condition.

Sometimes do..while() is used to create a scope that can be left early.
In such cases, the unreachable 'while()' test is not usually interesting
unless it actually does something that is observable.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203036 91177308-0d34-0410-b5e6-96231b3b80d8