[Clang][DiagnosticSemaKinds] combine diagnostic texts

[Clang][DiagnosticSemaKinds] combine diagnostic texts

The diagnostic texts for warning on attributes that don't appear on the
initial declaration is generally useful.  We'd like to re-use it in
D106030, but first let's combine two that already are very similar so we
may re-use it a third time in that commit.

Also, fix a few places that were using notePreviousDefinition to point
to declarations, to instead use diag::note_previous_declaration.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D107613