Deprecate -Yinfer-debug

Since 4d6be05c2 it has been ignored, being subsumed by -Ytyper-debug.

This commit makes it an deprecated alias for -Ytyper-debug.

We'd be within our rights to remove it outright, but we've got
a nice deprecation mechanism for settings, so let's use that for
a release.
Merge pull request #2939 from retronym/topic/infer-debug

Deprecate -Yinfer-debug