Replace __pragma with _Pragma

[CdTCzech]

Description

Resolves #659

Checklist

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• Identifiers in product code changes are properly _Ugly as per
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[miscco]

From the comment in the issue:

Note: Standard _Pragma takes a string while non-Standard __pragma doesn't, as the existing code demonstrates.

[CdTCzech]

From the comment in the issue:

Note: Standard _Pragma takes a string while non-Standard __pragma doesn't, as the existing code demonstrates.

Oh, such a mistake :( I was so scared of my first pull request ever that I forgot to sync my changes properly...
EDIT: Fixed

[CaseyCarter]

I was so scared of my first pull request ever that I forgot to sync my changes properly...

No need to be scared: we save our soul-crushing toxicity for the second pull request. (Don't worry about the test failures; tr1/fstream1 is broken due to a race condition that is being fixed in #647.)

[cbezault]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[barcharcraz]

Thank you for the contribution!

[barcharcraz]

Unfortunately we had to revert this PR due to issues with _Pragma in 16.6 preview 2 under /Zc:preprocessor. :(

We'll revert the PR once that issue is fixed

[CdTCzech]

Unfortunately we had to revert this PR due to issues with _Pragma in 16.6 preview 2 under /Zc:preprocessor. :(

We'll revert the PR once that issue is fixed

I'm always glad to be a first revert of the repository 😄
Thanks for letting me know and hopefully it will go smoother with my second one!

[StephanTLavavej]

Not a problem with your work at all! 😸 In fact, your change has led to two concrete improvements: (1) we've updated our checklist for merging PRs, and (2) we were completely lacking STL test coverage for /Zc:preprocessor, both on GitHub and in our Microsoft-internal test harness. The only thing that identified this was range-v3 test coverage in the compiler tests, that happened to drag in the STL (specifically <atomic>) and use /Zc:preprocessor. I had totally forgotten that we were missing any use of this option in tests/std - we're usually very diligent about testing conformance modes like /permissive- and so forth. @barcharcraz is now working on adding proper test coverage, so when the compiler bug is fixed and ships in a public Preview, we'll be able to restore your change with confidence.