[-Wunreachable-code] Tweak heuristic for configuration values to include arithmetic operations involving sizeof(), but not raw integers.

This case was motivated by a false positive with the
llvm::AlignOf<> specialization in LLVM.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@203363 91177308-0d34-0410-b5e6-96231b3b80d8