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

[Balázs Benics via cfe-commits]

steakhal wrote:

I resign from the review. I don't think I'd oppose to any of the outlined options. I'd recommend to reach consensus and go with one solution; I care less about which solution to be picked.

This could have been a subject to the static analysis working group, but the upcoming meeting is way out in the future right now.

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