Demote the "Debug Info Version" module flag to llvm::Module::Warning

behavior on mismatch. The AutoUpgrader will drop incompatible debug info
any way and also emit a warning diagnostic for it.

rdar://problem/16926122

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209182 91177308-0d34-0410-b5e6-96231b3b80d8