Fix iostreams with imbued locales to print INFs correctly

[fsb4000]

Followup to #3868.

[StephanTLavavej]

I've pushed changes to:

• Use isfinite as suggested by @frederick-vs-ja, which is faster and simpler than testing isnan and isinf separately,
• Rename _Fput_v2 to _Fput_v3 because the sense of the bool is flipping (this isn't really necessary if _Fput_v2 never shipped for production usage, but it was the cleanest thing to do),
• Guard the test with TEST_CUSTOM_FACET, with a comment explaining why,
• Update Dev11_0496153_locale_ctor which had an equivalent guard that was phrased in a much more confusing manner. (Its comment previously explained the preprocessor logic in English, but not why it existed.)

[StephanTLavavej]

D'oh, I checked std but didn't rerun the libcxx failures.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

♾️ 🌐 🎉