BitcodeReader: Use IsDistinct in upgrade from DIModule
BitcodeReader: Use IsDistinct in upgrade from DIModule

Support bitcode upgrade from DIModule, as a fixup for the cherry-pick of
r267268 ("BitcodeReader: Consistently use IsDistinct, NFC").

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm