DiagnosticIDs: Forbid Diag ID from being valid

Diag ID is used throughout clang as a sentinel id meaning "this is an
invalid diagnostic id."  Confusingly, Diag ID maps to a valid, usable,
diagnostic id.  Instead, start diagnostic ids at ID one.

Incidently, remove an unused element from StaticDiagInfo.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186760 91177308-0d34-0410-b5e6-96231b3b80d8