[llvm] [llvm-link] Add the `--ignore-if-conflict` option (PR #75696)

[Teresa Johnson via llvm-commits]

teresajohnson wrote:

I'm confused by the motivation. The description says it is about overriding compiler builtins, but the source changes don't do anything specific regarding builtins. The source changes just say ignore any already defined symbols in the given input files. Normally one would give symbols that can be overridden weak linkage.

https://github.com/llvm/llvm-project/pull/75696