Fix issue 21000 - allow use of std{in,out,err} with -preview=noshared…

[atilaneves]

…access

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21000	enhancement	-preview=nosharedaccess precludes use of stdin,stdout,stderr

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#8562"

[atilaneves]

I don't know how to prove this works other than to say I tried it manually. I can't add the preview flag since this particular issue was covering up many more.

[CyberShadow]

There are some stand-alone tests in the test directory, maybe that would be a good place?

[RazvanN7]

ping @atilaneves

[RazvanN7]

There are 2 ways we can go about this:

(1) add a test in the tests directory and compile it with -preview=nosharedaccess; this seems like an unpopular path since there are only 3 tests in that directory.

(2) continue fixing this up and put this test in a unittest once we are able to compile phobos with -preview=nosharedaccess`; I am inclined to go down this route as long as you are pursuing it @atilaneves .

Let me know what you prefer.

[atilaneves]

For now I'd prefer to put it in the test directory, but I don't see anything in the Makefile that runs those except for explicitly running the betterC tests.

TBH I think that this is overkill given that this is necessary and I'd be in favour of merging as-is.