[lld] [llvm] [lld][DebugInfo/BTF] Add BTF section merging and deduplication (PR #183915)

[Can H. Tartanoglu via llvm-commits]

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caniko wrote:

`Config::btfMerge` is immediately after `branchToBranch`, among the b-options.

Source: https://github.com/llvm/llvm-project/blob/3362413f61ac2e1718fd8ecdb9ef46b4dcc452ee/lld/ELF/Config.h. These file blobs are identical to tested head `ba019710f5de8180761c40fa1e90dff5f578ecfd`: [premerge checks](https://github.com/llvm/llvm-project/actions/runs/23135916624) and [formatter](https://github.com/llvm/llvm-project/actions/runs/23135916579) passed there. Head `3362413f61ac2e1718fd8ecdb9ef46b4dcc452ee` only strengthens a separate COMDAT assertion; its new test qualification and the end-to-end kernel receipts remain pending.

https://github.com/llvm/llvm-project/pull/183915