Remove `LlvmArchiveBuilder` after a few months if no regression is found

[jieyouxu]

Support reading thin archives in ArArchiveBuilder #128936 switches from using LlvmArchiveBuilder to ArArchiveBuilder.

[pirama-arumuga-nainar]

In Android, we have a CI that builds AOSP with ToT clang toolchain and the latest Android Rust toolchain (currently 1.83). We use LTO for both clang and rust. If a C++ object is added to an .rlib, rustc complains about mismatched LLVM versions

LLVM error: Invalid attribute group entry (Producer: 'LLVM20.0.0' Reader: 'LLVM 19.0.1)

even though the build step is just packaging the object files and not merging the LLVM IR modules. The final link is performed by the lld from ToT llvm-project, which can correctly handle LLVM IR from different producers.

@pcc added a change to LLVM ArchiveBuilder that only ignores this error when building a static lib: llvm/llvm-project#96848. It'd be great if ArArchiveBuilder also has the ability to ignore unknown bitcode when creating rlibs.

(cc: @stephenhines @maurer)

[bjorn3]

Opened #137017 to turn the error into a warning.