Fix Issue 22117 - Can't store scope pointer in a SumType

[pbackus]

Previously, the assignment of the local variable 'newStorage' to the
longer-lived member variable 'storage' caused scope inference to
(correctly) fail.

newStorage was necessary to work around issues 21229 and 22118. Since
those issues have been fixed, newStorage can be safely removed.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	22117	enhancement	Can't store scope pointer in a SumType

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#8208"

[pbackus]

Auto-tester failure is in druntime's forkgc2 test on 64-bit Darwin:

forkgc2(91895,0x700000104000) malloc: *** error for object 0x7fe3c3403130: double free

Probably a real bug worth investigating, but IMO it should not block this PR, so I'm force-pushing to restart the tests.