Turn enforce into an eponymous template + undocument enforceEx

[wilzbach]

Resubmission of the reverted #6086.
We need to figure out why this broke the DMD testsuite on SemaphoreCi and CircleCi.

test_results/d_do_test: symbol lookup error: test_results/d_do_test: undefined symbol: _D3std9exception__T7enforceZ__TQmTbZQrFNaNfbLAxaAyamZb
Makefile:209: recipe for target 'test_results/runnable/helloUTF16BE.d.out' failed

See also:

• Turn enforce into an eponymous template + undocument enforceEx #6086 (original PR)
• https://github.com/dlang/phobos/pull/60130 (revert)

[dlang-bot]

Thanks for your pull request, @wilzbach!

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

[wilzbach]

The SemaphoreCI problem has been found and fixed (see dlang/dmd@1e81c60).
So we can put this back in again :)

@CyberShadow around for a quick merge?

[wilzbach]

Jenkins didn't send a proper success code, but it passed:

Everything else is passing too:

So I'm merging this manually.