DebugInfo: Put each kind of constant (form, attribute, tag, etc) into its own enum for ease of use.

This allows various variables to be more self-documenting and easier to
debug by being of specific types without overlapping enum values.

Precommit review by Eric Christopher.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193091 91177308-0d34-0410-b5e6-96231b3b80d8