[clang] [clang][CrossTU] Indicate weak link attribute in ExternalDefMap. (PR #204188)

[Balázs Benics via cfe-commits]

steakhal wrote:

PS: I could not drop my changes requested - so I requested another round of review from myself. I won't come back to this thread, so don't wait on my review.

https://github.com/llvm/llvm-project/pull/204188