Sema: Cleanup and simplify anonymous union diagnostics

The determination of which diagnostics would be issued for certain
anonymous unions started to get a little ridiculous.  Clean this up by
inverting the condition-tree's logic from dialect -> issue to
issue -> diagnostic.

As part of this cleanup, move ext_c99_flexible_array_member from
DiagnosticParseKinds.td to DiagnosticSemaKinds.td because it's driven by
Sema, not Parse.

Also, the liberty was taken to edit ext_c99_flexible_array_member to
match other, similar, diagnostics.