Place for shared diagnostic utility function?

Clang style question: Now that many places that used to be
{struct|class} in DiagnosticSemaKinds.td are {struct|interface|class},
I've been writing short static getXDiagFromTagKind functions to return
the diag index given a TTK value. There are now 3 sema and 1 ast file
that require such a function; is there a good place to share it, or is
it preferred to include appropriate copies near their use for locality?

It might actually be simpler to reorder the TTK enums and then just use them DIRECTLY in diagnostics. 'struct', 'class', and 'interface' would all belong together. 'union' sometimes also belongs there. And 'enum' would still be last.

What does the list think?