Otherwise, this uniqueness check will unexpectedly succeed when tests
run in optimize mode and 'c' is destroyed early:
let c = C()
var b = B(native: c, isFlagged: flag)
//...
expectFalse(b.isUniquelyReferencedUnflaggedNative())
// Keep 'c' alive for the isUniquelyReferenced check above.
_fixLifetime(c)
Fixes rdar://72957398 ([CanonicalOSSA] Add a _fixLifetime to
stdlib/BridgeStorage.swift test.)
