[docs] Regenerate DiagnosticsReference.rst

It looks like it has been a while since the checked-in version of
DiagnosticsReference.rst was regenerated.  I realise there probably
isn't any expectation that the checked-in version is kept up-to-date,
but now that the project is on github and the rst can be viewed directly
from the repo's web interface, it seemed worth having something a bit
more recent.

Differential Revision: https://reviews.llvm.org/D79236