Don't destroy a weak pointer
Don't destroy a weak pointer

The state of having a weak pointer in BuildKey was previously added but the check before destroying wasn't included.

rdar://54298130

apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild