[llvm] [cmake] Add a new flag MERGE_INTO_LLVM_DYLIB to llvm_add_library (PR #158023)

[Petr Hosek via llvm-commits]

petrhosek wrote:

> I really do not like this direction. We already have the proper tools for this in CMake: Object Libraries. We should explicitly create the object library which will effectively "merge" the libraries. All of this custom scaffolding in LLVM makes it harder for newcomers to the project and prevents the use of the existing documentation from being useful.

I fully agree, this change is adding more complexity making it harder to understand and maintain the LLVM build system. We should be instead looking for opportunities to simplify our build system by leveraging standard CMake mechanisms like object libraries which seems to be the best fit for what you're trying to achieve with this change.

https://github.com/llvm/llvm-project/pull/158023