Triple-mirror visualizer changes

[StephanTLavavej]

#5176 and #5177 will need to be triple-mirrored to the VS repo in order to take effect.

[CaseyCarter]

VS-PR-603554 "Update STL.natvis from GitHub" will resolve this.

[CaseyCarter]

I've merged VS-PR-603554. Visual Studio changes are currently flowing into 17.14p1, which agrees with our Changelog entries for #5176 and #5177, so I'm leaving the Changelog alone.