[CanonicalOSSA] fix corelibs-foundation XMLTest
[CanonicalOSSA] fix corelibs-foundation XMLTest

Fix weak reference lifetime.

XMLParser has a weak delegate. OSSA optimization destroys the delegate
before this parser test runs.

Fixes rdar://73046709 ([CanonicalOSSA] corelibs-foundation XMLTest;
fix weak reference lifetime)

apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation