STL: Remove !_HAS_EXACT_COMPOUND_REQUIREMENT fallback

[S. B. Tam (cpplearner)]

Description

Fixes #299.

Checklist

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• Identifiers in product code changes are properly _Ugly as per
  https://eel.is/c++draft/lex.name#3.1 or there are no product code changes.
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  or trivially copyable, etc.).
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[Microsoft Contribution License Agreements (msftclas)]

All CLA requirements met.

[Casey Carter (CaseyCarter)]

This all looks correct - congratulations on submitting the first all-red PR! I'm happy to merge this contingent on tests passing. (Which they should unless there's a typo in here that I missed: we currently test concepts only with MSVC and it uses the _HAS_EXACT_COMPOUND_REQUIREMENT paths.)

[Stephan T. Lavavej (StephanTLavavej)]

Looks good to me. I verified that there are no occurrences of this macro remaining in the product code, and I also searched our (MSVC-internal but not for much longer) test code and found no occurrences.

[Stephan T. Lavavej (StephanTLavavej)]

I'm porting this to our Microsoft-internal MSVC repo now, for a full build and test run.

[Stephan T. Lavavej (StephanTLavavej)]

This passed our tests, so I've merged it for the VS 2019 16.5 Preview 2 release. Thanks again for the cleanup, and congratulations on your first commit in the STL repo!