Fix slow compilation when using std.regex

[DmitryOlshansky]

Taking this short program as an example:

import std.regex, std.stdio;

void main(string[] argv) {
	auto re = regex(argv[1]);
	auto matches = matchFirst(argv[2], re);
	writeln(matches);
}

Before:

time dmd re.d

real	0m2,404s
user	0m2,205s
sys	0m0,200s

After:

time dmd re.d

real	0m1,198s
user	0m1,103s
sys	0m0,096s

I still think the compile time is too big, maybe there is another CTFE-heavy construction hiding in there.

[dlang-bot]

Thanks for your pull request, @DmitryOlshansky!

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Testing this PR locally

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dub run digger -- build "master + phobos#8628"

[DmitryOlshansky]

@RazvanN7 I don't get cirrus-ci failing on some UDA test in std.traits that shouldn't be affected by my code at all. Any ideas?

[RazvanN7]

You should probably rebase to the latest version of phobos, that will most likely fix it.

[DmitryOlshansky]

In general I approve this pr