Hook up -Wunused-value to the unused expression warning. This is

Hook up -Wunused-value to the unused expression warning.  This is
useful for macro-heavy codebases that tend to trigger this warning a
lot.

Note that a make clean is required to get the option working; the
dependencies for DiagnosticGroups.inc appear to be broken.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@69564 91177308-0d34-0410-b5e6-96231b3b80d8