Use the ShowInSystemHeader bit consistently for all diagnostics

By describing system header suppressions directly in tablegen we eliminate
special cases in getDiagnosticSeverity().

Dropping the reliance on builtin diagnostic classes when mapping also gets us
closer to the goal of reusing the diagnostic machinery for custom diagnostics.

No change in functionality.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211023 91177308-0d34-0410-b5e6-96231b3b80d8