[Frontend] Disable value name discarding for all sanitizers.

ASan also relies on names on allocas and will emit unhelpful output if
they're not present. Just force-enable value names for now. Should
unbreak release builds of asan.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@263429 91177308-0d34-0410-b5e6-96231b3b80d8