[llvm] [Linker] Propagate `nobuiltin` attributes when linking known libcalls (PR #89431)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> I don't think this works without a module flag because the user might switch the order; linking the application into libc, or linking something else later into the application with libc. Either way, we wouldn't link in libcalls but we already had them. Still we can't perform libcall reasoning on the new functions.

Doesn't  this cover that case? The test file links the application and `libc` definition in either order. Both cases put `nobuiltin` on all the resulting functions.

https://github.com/llvm/llvm-project/pull/89431