Clang diagnostics should not start with a capital letter or use trailing punctuation (https://clang.llvm.org/docs/InternalsManual.html#the-format-string), but quite a few driver diagnostics were not following this advice. This corrects the grammar and punctuation to improve consistency, but does not change the circumstances under which the diagnostics are produced.
(This hopefully shouldn't require a considerable amount of review, but I wanted the CI pipeline to run over the changes to hopefully catch any tests that don't run on Windows.)
Would be more consistent with the options mixed with C/C++ mode/etc.