DiagnosticVerifier: Allow specifying locations vs. ranges in fix-it verifications

[AnthonyLatsis]

This spares us from needless repetition in the bounds of insertion fix-its.

[AnthonyLatsis]

@swift-ci please smoke test macOS

[AnthonyLatsis]

@swift-ci please smoke test macOS

[rintaro]

Sorry, but I personally don't think this is a good idea. {{5=a}} looks like replacing the character at column 5 with a at a glance.
Are there particular cases we strongly want this change?

[AnthonyLatsis]

It reduces duplication, and therefore potential typing errors, and makes it easier to read through numerous verified fix-its that benefit from this change (#41198 has an example test file), especially the ones that use offsets like -1:1--1:1 (I prefer these for robustness). It sounds like an edge case for someone to mistake 5-6= for 5=. We could continue rendering a range to clarify in case that happens. WDYT?

[rintaro]

For making fix-it test cases, I usually add {{none}} to expected-error etc. so the verifier can suggest the "correct" expected fix-its. So I rarely type expected fix-its manually. (Of course, I always check the suggested fix-its are actually correct.)
As for readability, for me, since they are test cases, I'd more appreciate if all test cases are written in a consistent syntax.

[AnthonyLatsis]

{{none}} is convenient, but even if you use --verify-apply-fixits, you still have to add lines/offsets on your own. On the other hand, the force-verifying of lines that we discussed earlier would alleviate the latter to some extent. Anyway, thanks for the feedback!