[clang-tblgen][NFC] renames Diagnostic.Text to Diagnostic.Summary

The [Improving Clang's Diagnostics RFC][1] identifies eight broad fields
for Clang to surface, two of which are text-based. Since the current
diagnostics more closely map to the diagnostic summary (or headline), we
should rename them to ensure that there's no confusion when
Diagnostic.Reason is introduced in the near future.

[1]: https://discourse.llvm.org/t/rfc-improving-clang-s-diagnostics/62584

Reviewed By: aaron.ballman, erichkeane

Differential Revision: https://reviews.llvm.org/D135820