Add a debug info code generation level to the compile unit metadata

Add a debug info code generation level to the compile unit metadata
and update everything accordingly. This can be used to conditionalize
the amount of output in the backend based on the amount of debug
requested/metadata emission scheme by a front end (e.g. clang).

Paired with a commit to clang.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202332 91177308-0d34-0410-b5e6-96231b3b80d8