Code cleanups: remove workarounds, use braces

[StephanTLavavej]

• Fix <yvals_core.h>: Warnings C4472 and C4571 are removed in VS 2019 16.6 #433: Warnings C4472 and C4571 were removed in VS 2019 16.6.
• Fix Testing followup for P0722R3 Destroying delete #693: Remove workarounds now that we require VS 2019 16.7.
• Use braces to construct tags (especially _Iter_cat_t), function objects, and empty shared_ptrs.

In the libcxx skip lists, we'll inevitably need the "missing compiler features" sections again, so they're preserved here.

[CaseyCarter]

Thanks for making many of our parentheses more wiggly. (I didn't Changelog this, shout if you disagree.)