The serialized diagnostic format has some extra fields that we can
adopt for diagnostic groups. Specifically:
* Category: store the diagnostic group name here
* Flags: extend the hack used by educational notes of placing Markdown file paths here
The inclusion of the full diagnostic list (via DiagnosticList.h) in
DiagnosticGroups.h meant that touching any diagnostic caused most of
the world to rebuild, making me sad. Use forward declarations to limit
how much needs to be rebuilt when changing diagnostics.
This commit adds new compiler options -no-warning-as-error/-warning-as-error which allows users to specify behavior for exact warnings and warning groups.
