CodeGen: fix code model mapping

Large is CodeModel::Model::Large, not CodeModel::Model::Medium.  Thanks to
majnemer for pointing out the typo!  Its unclear how to test the mapped value in
the compiler, the tests already cover the driver side.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208335 91177308-0d34-0410-b5e6-96231b3b80d8