Revert "Diagnose -Wunused-value based on CFG reachability"

This reverts commit 63e0d038.

It causes test failures:

http://lab.llvm.org:8011/#/builders/119/builds/5612
https://logs.chromium.org/logs/fuchsia/buildbucket/cr-buildbucket/8835548361443044001/+/u/clang/test/stdout